Gas sensor

Abstract

The invention relates to a gas sensor. In a gas sensor provided with a temperature sensing element, the time for which the concentration of a gas to be detected can be detected is increased. The gas sensor (1) detects a detection voltage present at a connection point (N1) of the thermistor (Rd1) and the thermistor (Rd2) during a period T1 in which the thermistors (Rd1, Rd3) are heated to a first temperature region and the thermistors (Rd2, Rd4) are heated to a second temperature region, and detects a detection voltage present at a connection point (N1) of the thermistor (Rd1) and the thermistor (Rd2, Rd4) during a period T2 in which the thermistors (Rd1, Rd3) are heated to the second temperature region and the thermistors (Rd2, Rd4) are heated to the first temperature region. A detection voltage present at a connection point (N2) between the thermistor (Rd3) and the thermistor (Rd4) is detected.

Description

Technical Field

[0001] The present disclosure relates to a gas sensor, and in particular to a gas sensor having a temperature sensing element such as a thermistor. Background Art

[0002] Patent Document 1 discloses a gas sensor including two thermistors connected in series. The gas sensor described in Patent Document 1 heats one thermistor to a first temperature range and the other thermistor to a second temperature range during a first period, and heats one thermistor to the second temperature range and the other thermistor to the first temperature range during a second period. This allows the difference in thermal history between the two thermistors to match.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: International Publication No. 2020 / 031517 Summary of the Invention

[0006] Technical problem to be solved by the invention

[0007] However, the gas sensor described in Patent Document 1 cannot detect the concentration of the detection target gas during the second period.

[0008] This disclosure describes a technique for increasing the time during which the concentration of a detection target gas can be detected in a gas sensor including a temperature sensing element such as a thermistor.

[0009] Technical solutions to technical problems

[0010] A gas sensor according to one aspect of the present disclosure comprises: a first series circuit including first and second temperature sensing elements connected in series; a second series circuit including third and fourth temperature sensing elements connected in series; a first power supply circuit applying a voltage to the first series circuit; a second power supply circuit applying a voltage to the second series circuit; and a signal processing circuit detecting a first detection voltage present at a first connection point connecting the first temperature sensing element and the second temperature sensing element during a first period in which the first and third temperature sensing elements are heated to a first temperature range and the second and fourth temperature sensing elements are heated to a second temperature range, and detecting a second detection voltage present at a second connection point connecting the third temperature sensing element and the fourth temperature sensing element during a second period in which the first and third temperature sensing elements are heated to the second temperature range and the second and fourth temperature sensing elements are heated to the first temperature range, wherein the signal processing circuit calculates the concentration of the gas to be detected based on the first and second detection voltages.

[0011] Effects of the Invention

[0012] According to the present disclosure, a technology is provided for increasing the time during which the concentration of a detection target gas can be detected in a gas sensor including a temperature sensing element such as a thermistor. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 1 is a circuit diagram showing the configuration of a gas sensor 1 according to a first embodiment of the technology disclosed herein.

[0014] Figure 2 It is a schematic plan view for explaining the device structure of the sensor units S1 and S2 , and shows a portion related to the heater resistors MH1 and MH2 .

[0015] Figure 3 It is a schematic plan view for explaining the device structure of the sensor units S1 and S2 , and shows a portion related to thermistors Rd1 to Rd4 .

[0016] Figure 4 This is a schematic top view for explaining the device structure of the sensor parts S1 and S2. Figure 2 and Figure 3 Overlapping state.

[0017] Figure 5 It is a schematic plan view for explaining a modified example of the device structure of the sensor units S1 and S2 , and shows a portion related to thermistors Rd1 to Rd4 .

[0018] Figure 6 This is a flowchart for explaining the operation of the gas sensor 1 .

[0019] Figure 7 This is a timing chart for explaining the operation of the gas sensor 1 .

[0020] Figure 8 This is a flowchart for explaining the operation of a modified example of the gas sensor 1 .

[0021] Figure 9 (a) is a graph showing changes in the measurement result signal OUT obtained when there is an offset between the detection voltage Vgas1 appearing at the connection point N1 and the detection voltage Vgas1 appearing at the connection point N2. Figure 9 (b) is a graph showing changes in the average value of the measurement result signal OUT obtained in a measurement at a certain time and the measurement result signal OUT obtained in the measurement before that time.

[0022] Figure 10 1 is a circuit diagram showing the configuration of a gas sensor 2 according to a second embodiment of the technology disclosed herein.

[0023] Figure 11 This is a circuit diagram showing the configuration of a gas sensor 3 according to a third embodiment of the technology disclosed herein.

[0024] Figure 12 1 is a circuit diagram showing the configuration of a gas sensor 4 according to a fourth embodiment of the technology disclosed herein.

[0025] Figure 13 1 is a circuit diagram showing the configuration of a gas sensor 5 according to a fifth embodiment of the technology disclosed herein.

[0026] Figure 14 It is a schematic plan view for explaining the device structure of the sensor units S1 and S2 in the gas sensor 5 , and shows a portion related to thermistors Rd1 to Rd4 .

[0027] Explanation of symbols:

[0028] 1~5 gas sensors

[0029] 10Signal processing circuit

[0030] 11, 11a, 11b amplifiers

[0031] 12AD converter

[0032] 13DA converter

[0033] 14 Control circuit

[0034] 20 substrates

[0035] 20A, 20B incision

[0036] 21 Insulation film

[0037] 31~36 thermistor film

[0038] 41~48 relative electrodes

[0039] 51~59 terminal electrodes

[0040] MH1, MH2 heater resistor

[0041] N1, N2 connection points

[0042] Rd1~Rd4 thermistors

[0043] S1, S2 sensor part

[0044] SW1 to SW5 switches

[0045] Vcc1, Vcc2 power supply voltage

[0046] Vgas1, Vgas1a, Vgas1b, Vgas2, Vgas2a, Vgas2b detection voltage

[0047] Vmh1, Vmh2 heater voltage

[0048] Vref reference voltage DETAILED DESCRIPTION

[0049] Below, with reference to the attached Figure 1 Embodiments of the technology disclosed herein will be described in detail.

[0050] <First embodiment>

[0051] Figure 1 1 is a circuit diagram showing the configuration of a gas sensor 1 according to a first embodiment of the technology disclosed herein.

[0052] like Figure 1 As shown, the gas sensor 1 of the first embodiment includes sensor units S1 and S2 and a signal processing circuit 10. Although not particularly limited, the gas sensor 1 of this embodiment is a thermal conductivity gas sensor for detecting the concentration of CO2 gas in the atmosphere.

[0053] The sensor unit S1 includes thermistors Rd1, Rd3 and heater resistor MH1. The sensor unit S2 includes thermistors Rd2, Rd4 and heater resistor MH2. Thermistors Rd1 to Rd4 all have negative temperature coefficients of resistance. Thermistor Rd1 and thermistor Rd2 are connected in series, whereby thermistor Rd1 and thermistor Rd2 constitute a first series circuit. Thermistor Rd3 and thermistor Rd4 are connected in series, whereby thermistor Rd3 and thermistor Rd4 constitute a second series circuit. One end of thermistor Rd1 and one end of thermistor Rd2 are short-circuited at a connection point N1. One end of thermistor Rd3 and one end of thermistor Rd4 are short-circuited at a connection point N2. At Figure 1 In the example shown, the connection point N1 and the connection point N2 are short-circuited.

[0054] The other end of thermistor Rd1 is connected to a wiring line supplying power supply voltage Vcc1 via switch SW1. The other end of thermistor Rd2 is connected to a ground wiring line via switch SW3. The other end of thermistor Rd3 is connected to a ground wiring line via switch SW4. The other end of thermistor Rd4 is connected to a wiring line supplying power supply voltage Vcc2 via switch SW2. Power supply voltage Vcc1 and power supply voltage Vcc2 may be at the same level. Switches SW1 and SW3, the wiring line supplying power supply voltage Vcc1, and the ground wiring constitute a first power supply circuit that applies voltage to the first series circuit formed by thermistors Rd1 and Rd2. Switches SW2 and SW4, the wiring line supplying power supply voltage Vcc2, and the ground wiring constitute a second power supply circuit that applies voltage to the second series circuit formed by thermistors Rd3 and Rd4. In the first series circuit formed by thermistors Rd1 and Rd2, the connection relationship with power supply voltage Vcc1 and the ground wiring may be reversed. That is, the other end of thermistor Rd1 may be connected to the ground line via switch SW1, while the other end of thermistor Rd2 may be connected to the line supplying power supply voltage Vcc1 via switch SW3. Similarly, in the second series circuit formed by thermistors Rd3 and Rd4, the connection relationship with power supply voltage Vcc2 and the ground line may be reversed. That is, the other end of thermistor Rd3 may be connected to the line supplying power supply voltage Vcc2 via switch SW4, while the other end of thermistor Rd4 may be connected to the ground line via switch SW2.

[0055] The resistance values of thermistors Rd1 and Rd4, which are temperature sensing elements for detection, are designed to fall within a first resistance range when heated to a first temperature range (e.g., approximately 150°C). The first temperature range is, for example, a specified temperature range within a range of 100°C to 230°C, such as a temperature range around 150°C. The resistance values of thermistors Rd2 and Rd3, which are temperature sensing elements for reference, are designed to fall within a second resistance range when heated to a second temperature range (e.g., approximately 300°C). The second temperature range is, for example, a specified temperature range within a range of 250°C to 450°C, such as a temperature range around 300°C. The "temperature range" in this specification has a temperature width of, for example, less than 1°C. For example, the temperature range around 150°C may also be a range between 149.5°C and 150.5°C. In addition, for example, the temperature range around 300°C may also be a range between 299.5°C and 300.5°C. In this embodiment, the temperature of the second temperature range is higher than that of the first temperature range. The first and second resistor regions may partially overlap or coincide with each other. When the first and second resistor regions overlap or coincide with each other, the level of the detection voltage Vgas1 at the connection points N1 and N2 can be set to approximately Vcc1 / 2 or Vcc2 / 2, thereby achieving a wide dynamic range. The detection voltage Vgas1 at the connection points N1 and N2 is supplied to the signal processing circuit 10.

[0056] When thermistors Rd1 and Rd4, temperature-sensing elements used for detection, are heated to approximately 150°C and CO2 gas is present in the measurement atmosphere, the heat dissipation characteristics of thermistors Rd1 and Rd4 change depending on the CO2 gas concentration. This change manifests as a change in the temperature of thermistors Rd1 and Rd4, or in other words, a change in their resistance. Specifically, CO2 gas has a lower heat dissipation property than air. Therefore, the higher the CO2 gas concentration, the higher the temperature of thermistors Rd1 and Rd4. Therefore, for example, if the CO2 gas concentration in the measurement atmosphere is the same as the average atmospheric concentration, and thermistors Rd1 and Rd4 are heated to 150°C, the temperature of thermistors Rd1 and Rd4 will exceed 150°C if the CO2 gas concentration in the measurement atmosphere is higher than the average atmospheric concentration. As a result, the resistance of thermistors Rd1 and Rd4 decreases compared to when the CO2 gas concentration in the measurement atmosphere is the average atmospheric concentration.

[0057] On the other hand, even when CO₂ gas is present in the measurement atmosphere while thermistors Rd2 and Rd3, the reference temperature sensing elements, are heated to approximately 300°C, the heat dissipation characteristics of thermistors Rd2 and Rd3 barely change depending on the CO₂ gas concentration, and the temperatures of thermistors Rd2 and Rd3 barely change. Therefore, the change in resistance of thermistors Rd2 and Rd3 due to CO₂ gas concentration when heated to approximately 300°C is significantly less than the change in resistance of thermistors Rd1 and Rd4 due to CO₂ gas concentration when heated to approximately 150°C. The resistance of thermistors Rd2 and Rd3 due to CO₂ gas concentration also barely changes when heated to approximately 300°C. As a result, when thermistor Rd1 is heated to approximately 150°C and thermistor Rd2 is heated to approximately 300°C, a detection voltage Vgas1 corresponding to the CO₂ gas concentration in the measurement atmosphere appears at connection point N1. Similarly, when thermistor Rd4 is heated to approximately 150°C and thermistor Rd3 is heated to approximately 300°C, a detection voltage Vgas1 corresponding to the CO2 gas concentration in the measurement atmosphere appears at connection point N2. On the other hand, even if the measurement atmosphere contains another gas whose heat dissipation characteristics when thermistors Rd1-Rd4 are heated to approximately 150°C and approximately 300°C are not significantly different, the concentration of that gas will have little effect on the detection voltage Vgas1. This allows for selective detection of the CO2 gas concentration.

[0058] The signal processing circuit 10 includes an amplifier 11, an analog-to-digital converter (ADC) 12, a digital-to-digital converter (DAC) 13, and a control circuit 14. The amplifier 11 compares the detection voltage Vgas1 with a reference voltage Vref to generate an amplified detection voltage Vgas2. The amplifier 11 is, for example, a differential amplifier, and amplifies the difference between the detection voltage Vgas1 and the reference voltage Vref to generate and output the detection voltage Vgas2. The detection voltage Vgas2 is input to the AD converter 12. The AD converter 12 performs AD conversion on the detection voltage Vgas2 to generate a digital value, which is then supplied to the control circuit 14.

[0059] The control circuit 14 supplies the digital values of various control parameters to the DA converter 13. The DA converter 13 performs analog conversion on the digital values of the various control parameters to generate heater voltages Vmh1 and Vmh2 and a reference voltage Vref. Heater voltage Vmh1 is applied to heater resistor MH1, thereby heating thermistors Rd1 and Rd3 together. Heater voltage Vmh2 is applied to heater resistor MH2, thereby heating thermistors Rd2 and Rd4 together. Reference voltage Vref is also supplied to amplifier 11.

[0060] Figures 2 to 4 1 is a schematic top view for explaining the device structure of the sensor units S1 and S2. Figure 2 Indicates the part related to heater resistors MH1 and MH2. Figure 3 Indicates the part related to thermistors Rd1 to Rd4. Figure 4 It will Figure 2 and Figure 3 Overlapping graphs.

[0061] like Figures 2 to 4 As shown, sensor units S1 and S2 include a substrate 20, an insulating film 21 provided on the surface of substrate 20, heater resistors MH1 and MH2 provided on insulating film 21, thermistor films 31 and 33 provided at a position overlapping heater resistor MH1, thermistor films 32 and 34 provided at a position overlapping heater resistor MH2, a pair of opposing electrodes 41 and 42 in contact with thermistor film 31, a pair of opposing electrodes 43 and 44 in contact with thermistor film 32, a pair of opposing electrodes 45 and 46 in contact with thermistor film 33, and a pair of opposing electrodes 47 and 48 in contact with thermistor film 34. A first portion of thermistor film 31 located between the pair of opposing electrodes 41 and 42 and the opposing electrodes 41 and 42 constitute thermistor Rd1. A first portion of thermistor film 32 located between the pair of opposing electrodes 43 and 44 and the opposing electrodes 43 and 44 constitute thermistor Rd2. The first portion of the thermistor film 33 located between the pair of opposing electrodes 45 and 46 and the opposing electrodes 45 and 46 constitute thermistor Rd3. The first portion of the thermistor film 34 located between the pair of opposing electrodes 47 and 48 and the opposing electrodes 47 and 48 constitute thermistor Rd4.

[0062] The material of substrate 20 is not particularly limited, as long as it has suitable mechanical strength and is suitable for microfabrication such as etching. Silicon single crystal substrates, sapphire single crystal substrates, ceramic substrates, quartz substrates, glass substrates, and the like can be used. Substrate 20 and insulating film 21 are removed at cutouts 20A and 20B, creating a cavity in these areas. Substrate 20 is removed in the area surrounded by cutouts 20A and 20B, and heater resistors MH1 and MH2 and thermistors Rd1 to Rd4 are supported by the thin insulating film 21. This diaphragm structure suppresses the conduction of heat generated by heater resistors MH1 and MH2 to substrate 20.

[0063] The heater resistors MH1 and MH2 are made of a conductive material, and a metal material made of a material with a relatively high melting point can be used, such as molybdenum (Mo), platinum (Pt), gold (Au), tungsten (W), tantalum (Ta), palladium (Pd), iridium (Ir), nickel (Ni), chromium (Cr), or an alloy containing any two or more of these. Figure 2As shown, one end of heater resistor MH1 is connected to terminal electrode 51, and the other end of heater resistor MH1 is connected to terminal electrode 53. In addition, one end of heater resistor MH2 is connected to terminal electrode 52, and the other end of heater resistor MH2 is connected to terminal electrode 53. Heater resistors MH1 and MH2 may also have a meandering shape.

[0064] Thermistor films 31-34 are made of a material having a negative temperature coefficient of resistance, such as a composite metal oxide, amorphous silicon, polycrystalline silicon, or germanium. Opposing electrodes 41 and 42 in contact with thermistor film 31 are connected to terminal electrodes 54 and 56, respectively. Thus, terminal electrode 54 and terminal electrode 56 are connected via thermistor film 31 located between opposing electrodes 41 and 42. Opposing electrodes 43 and 44 in contact with thermistor film 32 are connected to terminal electrodes 56 and 55, respectively. Thus, terminal electrode 55 and terminal electrode 56 are connected via thermistor film 32 located between opposing electrodes 43 and 44. Opposing electrodes 45 and 46 in contact with thermistor film 33 are connected to terminal electrodes 57 and 59, respectively. Thus, terminal electrode 57 and terminal electrode 59 are connected via thermistor film 33 located between opposing electrodes 45 and 46. Opposing electrodes 47 and 48 in contact with thermistor film 34 are connected to terminal electrodes 59 and 58, respectively. As a result, the terminal electrode 58 and the terminal electrode 59 are connected via the thermistor film 34 located between the counter electrodes 47 and 48 .

[0065] Terminal electrode 54 is connected to a line supplying power supply voltage Vcc1 via switch SW1. Terminal electrode 55 is connected to a ground line via switch SW3. Terminal electrode 58 is connected to a line supplying power supply voltage Vcc2 via switch SW2. Terminal electrode 57 is connected to a ground line via switch SW4. Switches SW1 to SW4 may also be part of signal processing circuit 10. Terminal electrode 56 constitutes connection point N1. Terminal electrode 59 constitutes connection point N2.

[0066] like Figure 3As shown, the counter electrodes 41, 42 and the counter electrodes 47, 48 are intermeshed like comb teeth, while the counter electrodes 43, 44 and the counter electrodes 45, 46 are linearly opposed to each other. Consequently, the relative length of the counter electrodes 41, 42 and the counter electrodes 47, 48 is longer than the relative length of the counter electrodes 43, 44 and the counter electrodes 45, 46. Furthermore, the inter-electrode distance (relative width) of the counter electrodes 41, 42 and the counter electrodes 47, 48 is shorter than the inter-electrode distance (relative width) of the counter electrodes 43, 44 and the counter electrodes 45, 46. As a result, the resistivities of the thermistor films 31 to 34 are substantially the same. When the thermistor films 31 to 34 are heated to substantially the same temperature, the resistance value of the thermistor film 31 located between the opposing electrode 41 and the opposing electrode 42, and the resistance value of the thermistor film 34 located between the opposing electrode 47 and the opposing electrode 48 are lower than the resistance value of the thermistor film 32 located between the opposing electrode 43 and the opposing electrode 44, and the resistance value of the thermistor film 33 located between the opposing electrode 45 and the opposing electrode 46. This is because, as mentioned above, the resistance values of thermistors Rd1 and Rd4, the temperature-sensing elements used for detection, are designed to enter the first resistance range when heated to approximately 150°C. In contrast, the resistance values of thermistors Rd2 and Rd3, the temperature-sensing elements used for reference, are designed to enter the second resistance range when heated to approximately 300°C. Therefore, in order to make the first resistance range and the second resistance range overlap or match, the resistance values of thermistors Rd1 and Rd4 need to be designed to be lower than the resistance values of thermistors Rd2 and Rd3 at the same temperature.

[0067] In addition, Figure 3 In the example shown, the thermistor film 31 and the thermistor film 33 are separated, and the thermistor film 32 and the thermistor film 34 are separated, but the thermistor film 31 and the thermistor film 33 may be separated. Figure 5 As shown in the example, a common thermistor film 35 is used for the counter electrodes 41, 42, 45, and 46, and a common thermistor film 36 is used for the counter electrodes 43, 44, 47, and 48. In this case, a first portion of the thermistor film 35 located between the pair of counter electrodes 41 and 42 and the counter electrodes 41 and 42 constitute thermistor Rd1, a first portion of the thermistor film 36 located between the pair of counter electrodes 43 and 44 and the counter electrodes 43 and 44 constitute thermistor Rd2, a second portion of the thermistor film 35 located between the pair of counter electrodes 45 and 46 and the counter electrodes 45 and 46 constitute thermistor Rd3, and a second portion of the thermistor film 36 located between the pair of counter electrodes 47 and 48 and the counter electrodes 47 and 48 constitute thermistor Rd4.

[0068] Next, the operation of the gas sensor 1 according to the first embodiment will be described.

[0069] Figure 6 Flowchart for explaining the operation of the gas sensor 1. Figure 7 This is a timing chart for explaining the operation of the gas sensor 1 .

[0070] First, the signal processing circuit 10 included in the gas sensor 1 obtains a temperature signal T indicating the temperature of the measurement atmosphere (step 100) and calculates a correction value based on the temperature signal T (step 101). The correction value is used to cancel the offset of the heater voltages Vmh1 and Vmh2 and the reference voltage Vref caused by the temperature of the measurement atmosphere.

[0071] Next, the signal processing circuit 10 performs operations during period T1. During period T1, the signal processing circuit 10 first outputs heater voltages Vmh1 and Vmh2 corrected based on the correction value (step 102). During period T1, heater resistors MH1 and MH2 are heated so that thermistors Rd1 and Rd3 reach approximately 150°C, and thermistors Rd2 and Rd4 reach approximately 300°C. For example, heater voltages Vmh1 and Vmh2 are set so that, when the CO2 gas concentration in the measurement atmosphere is the same as the average concentration in the atmosphere, thermistors Rd1 and Rd3 are heated to 150°C and thermistors Rd2 and Rd4 are heated to 300°C. In this state, the signal processing circuit 10 turns on switches SW1 and SW3 and turns off switches SW2 and SW4 (step 103). As a result, a detection voltage Vgas1 corresponding to the CO2 gas concentration in the measurement atmosphere appears at connection point N1. The detection voltage Vgas1 is received by the signal processing circuit 10 (step 104). The control circuit 14 included in the signal processing circuit 10 calculates a measurement result signal OUT indicating the CO2 gas concentration and outputs it externally (step 105). During period T1, thermistors Rd3 and Rd4 are also heated. Since switches SW2 and SW4 are off during period T1, thermistors Rd3 and Rd4 do not affect the detection voltage Vgas1.

[0072] Next, the signal processing circuit 10 performs operations during period T2. During period T2, the signal processing circuit 10 first outputs heater voltages Vmh1 and Vmh2 corrected based on the correction value (step 106). During period T2, the heater resistors MH1 and MH2 are heated so that thermistors Rd1 and Rd3 reach approximately 300°C and thermistors Rd2 and Rd4 reach approximately 150°C. For example, the heater voltages Vmh1 and Vmh2 are set so that, when the CO2 gas concentration in the measurement atmosphere is the same as the average concentration in the atmosphere, thermistors Rd1 and Rd3 are heated to 300°C and thermistors Rd2 and Rd4 are heated to 150°C. In this state, the signal processing circuit 10 turns off switches SW1 and SW3 and turns on switches SW2 and SW4 (step 107). As a result, a detection voltage Vgas1 corresponding to the CO2 gas concentration in the measurement atmosphere appears at the connection point N2. The detection voltage Vgas1 is received by the signal processing circuit 10 (step 108). The control circuit 14 included in the signal processing circuit 10 calculates a measurement result signal OUT indicating the CO2 gas concentration and outputs it externally (step 109). During period T2, thermistors Rd1 and Rd2 are also heated. During period T2, switches SW1 and SW3 are off, so thermistors Rd1 and Rd2 do not affect the detection voltage Vgas1.

[0073] By repeating the above-described steps 102 to 109 until the measurement is completed, the measurement result signal OUT can be periodically acquired (step 110 ).

[0074] Thus, the gas sensor 1 of the first embodiment alternately repeats period T1, in which thermistors Rd1 and Rd3 are heated to approximately 150°C and thermistors Rd2 and Rd4 are heated to approximately 300°C, and period T2, in which thermistors Rd1 and Rd3 are heated to approximately 300°C and thermistors Rd2 and Rd4 are heated to approximately 150°C. This allows the measurement result signal OUT to be obtained during both period T1 and period T2, while aligning the thermal histories of thermistors Rd1 and Rd2, and the thermal histories of thermistors Rd3 and Rd4. However, alternating periods T1 and T2 is not essential; it is also possible to perform the operation during period T1 multiple times and then perform the operation during period T2 multiple times.

[0075] Figure 8 This is a flowchart for explaining the operation of a modified example of the gas sensor 1. Figure 6 The same actions as described above are denoted by the same symbols.

[0076] First, after acquiring the temperature signal T (step 100) and calculating the correction value (step 101), the signal processing circuit 10 performs the first operation of period T1. The first operation of period T1 (steps 202 to 205) is basically the same as steps 102 to 105 described above, but in step 205, only the measurement result signal OUT is calculated, and the measurement result signal OUT is not output externally.

[0077] Next, the signal processing circuit 10 performs operations during period T2. The operations during period T2 are basically the same as those in steps 106 to 109 described above, but in step 109, which outputs the measurement result signal OUT, the average value of the measurement result signal OUT calculated in step 205 and the measurement result signal OUT calculated in step 109 is output to the outside.

[0078] Next, the signal processing circuit 10 performs operations during period T1. The operations during period T1 are basically the same as those in steps 102 to 105 described above. However, in step 105, which outputs the measurement result signal OUT, the average value of the measurement result signal OUT calculated in step 109 and the measurement result signal OUT calculated in step 105 is output to the outside.

[0079] In this way, Figure 8 In the example shown, since the average value of the measurement result signal OUT obtained in the measurement at a certain time and the measurement result signal OUT obtained in the measurement before this time is output to the outside, for example, Figure 9 As shown in (a), even when there is an offset between the detection voltage Vgas1 present at the connection point N1 and the detection voltage Vgas1 present at the connection point N2, Figure 9 As shown in (b), the measurement result signal OUT actually output does not exhibit any offset. Furthermore, while the above example outputs a moving average of two measurement result signals OUT, a moving average of more measurement result signals OUT (e.g., the measurement result signal OUT obtained in the current measurement and the measurement result signals OUT obtained in multiple previous measurements) may be calculated and output.

[0080] <Second embodiment>

[0081] Figure 10 1 is a circuit diagram showing the configuration of a gas sensor 2 according to a second embodiment of the technology disclosed herein.

[0082] like Figure 10As shown in FIG. 1 , the gas sensor 2 of the second embodiment differs from the gas sensor 1 of the first embodiment in that switches SW3 and SW4 are removed, and the other ends of the thermistor Rd2 and the other ends of the thermistor Rd3 are directly connected to the ground wiring. The rest of the basic structure is the same as that of the gas sensor 1 of the first embodiment. Therefore, identical elements are denoted by identical reference numerals, and duplicate descriptions are omitted.

[0083] The operation of the gas sensor 2 according to the second embodiment is similar to that of the gas sensor 1 according to the first embodiment, except that switches SW3 and SW4 are not controlled. In the gas sensor 2 according to the second embodiment, since switches SW3 and SW4 are not present, during period T1, connection point N2 is connected to the ground line via reference thermistor Rd3, and during period T2, connection point N1 is connected to the ground line via reference thermistor Rd2. However, while thermistor Rd3 is heated to approximately 150°C during period T1 and thermistor Rd2 is heated to approximately 150°C during period T2, the resistance values of reference thermistors Rd2 and Rd3 are designed to fall within a predetermined resistance range when heated to approximately 300°C. Therefore, the resistance values of thermistors Rd2 and Rd3 heated to approximately 150°C are sufficiently high to have a negative temperature coefficient of resistance. Therefore, the effects of thermistor Rd3 during period T1 and thermistor Rd2 during period T2 are negligible.

[0084] As described above, the gas sensor 2 according to the second embodiment can further simplify the circuit scale because the switches SW3 and SW4 are omitted.

[0085] <Third embodiment>

[0086] Figure 11 This is a circuit diagram showing the configuration of a gas sensor 3 according to a third embodiment of the technology disclosed herein.

[0087] like Figure 11As shown, in the gas sensor 3 of the third embodiment, switches SW1 to SW4 are eliminated. Therefore, the other end of thermistor Rd1 is directly connected to the wiring supplying power supply voltage Vcc1, the other ends of thermistors Rd2 and Rd3 are directly connected to the ground wiring, and the other end of thermistor Rd4 is directly connected to the wiring supplying power supply voltage Vcc2. Furthermore, the gas sensor 3 of the third embodiment includes a switch SW5 that connects one of connection point N1 and connection point N2 to amplifier 11. Switch SW5 is controlled by signal processing circuit 10. When connection point N1 is selected, detection voltage Vgas1a at connection point N1 is supplied to amplifier 11 as detection voltage Vgas1. When connection point N2 is selected, detection voltage Vgas1b at connection point N2 is supplied to amplifier 11 as detection voltage Vgas1. Signal processing circuit 10 controls switch SW5 to select connection point N1 during period T1 and to select connection point N2 during period T2. The other basic configurations are the same as those of the gas sensor 1 according to the first embodiment, and therefore the same elements are denoted by the same reference numerals, and redundant descriptions are omitted.

[0088] Thus, the gas sensor 3 of the third embodiment does not short-circuit the connection point N1 and the connection point N2 and is exclusively connected to the amplifier 11 via the switch SW5. Therefore, the detection voltage Vgas1a appearing at the connection point N1 and the detection voltage Vgas1b appearing at the connection point N2 do not affect each other.

[0089] <Fourth embodiment>

[0090] Figure 12 1 is a circuit diagram showing the configuration of a gas sensor 4 according to a fourth embodiment of the technology disclosed herein.

[0091] like Figure 12As shown, in the gas sensor 4 of the fourth embodiment, the detection voltage Vgas1a present at the connection point N1 is supplied to an amplifier 11a, and the detection voltage Vgas1b present at the connection point N2 is supplied to an amplifier 11b. The amplifier 11a is, for example, a differential amplifier, and generates an amplified detection voltage Vgas2a by comparing the detection voltage Vgas1a with a reference voltage Vref. The amplifier 11b is, for example, a differential amplifier, and generates an amplified detection voltage Vgas2b by comparing the detection voltage Vgas1b with a reference voltage Vref. The detection voltages Vgas2a and Vgas2b are input to an AD converter 12. The AD converter 12 performs AD conversion on the detection voltages Vgas2a and Vgas2b to generate digital values, which are then supplied to a control circuit 14. The remaining basic structure is the same as that of the gas sensor 3 of the third embodiment. Therefore, identical elements are denoted by identical reference numerals, and duplicate descriptions are omitted.

[0092] Thus, the gas sensor 4 of the fourth embodiment does not short-circuit the connection point N1 and the connection point N2, and is connected to different amplifiers 11a and 11b respectively. Therefore, the detection voltage Vgas1a appearing at the connection point N1 and the detection voltage Vgas1b appearing at the connection point N2 do not affect each other.

[0093] <Fifth embodiment>

[0094] Figure 13 1 is a circuit diagram showing the configuration of a gas sensor 5 according to a fifth embodiment of the technology disclosed herein.

[0095] like Figure 13 As shown, the gas sensor 5 of the fifth embodiment differs from the gas sensor 1 of the first embodiment in that the switches SW1 and SW2 are removed, the other end of the thermistor Rd1 is directly connected to the wiring supplying the power supply voltage Vcc1, and the other end of the thermistor Rd4 is directly connected to the wiring supplying the power supply voltage Vcc2, and the thermistor films 31 to 34 are made of a material having a positive temperature coefficient of resistance. In addition, in the gas sensor 5 of the fifth embodiment, as shown in FIG. Figure 14As shown, the counter electrodes 43, 44 and the counter electrodes 45, 46 are intermeshed like comb teeth, while the counter electrodes 41, 42 and the counter electrodes 47, 48 are linearly opposed to each other. Consequently, the relative length of the counter electrodes 43, 44 and the counter electrodes 45, 46 is longer than the relative length of the counter electrodes 41, 42 and the counter electrodes 47, 48. Furthermore, the inter-electrode distance (relative width) of the counter electrodes 43, 44 and the counter electrodes 45, 46 is shorter than the inter-electrode distance (relative width) of the counter electrodes 41, 42 and the counter electrodes 47, 48. Therefore, when the resistivity of thermistor films 31 to 34 is substantially the same and thermistor films 31 to 34 are heated to substantially the same temperature, the resistance value of thermistor film 31 located between the opposing electrodes 41 and 42 and the resistance value of thermistor film 34 located between the opposing electrodes 47 and 48 are higher than the resistance value of thermistor film 32 located between the opposing electrodes 43 and 44 and the resistance value of thermistor film 33 located between the opposing electrodes 45 and 46. The rest of the basic structure is the same as that of the gas sensor 1 according to the first embodiment. Therefore, the same reference numerals are used to designate the same elements, and duplicate descriptions will be omitted.

[0096] The operation of the gas sensor 5 of the fifth embodiment is the same as that of the gas sensor 1 of the first embodiment, except that switches SW1 and SW2 are not controlled. Since the gas sensor 5 of the fifth embodiment does not have switches SW1 and SW2, during period T1, the connection point N2 is connected to the wiring supplying the power supply voltage Vcc2 via the detection thermistor Rd4, and during period T2, the connection point N1 is connected to the wiring supplying the power supply voltage Vcc1 via the detection thermistor Rd1. However, while thermistor Rd4 is heated to approximately 300°C during period T1 and thermistor Rd1 is heated to approximately 300°C during period T2, the resistance values of the detection thermistors Rd1 and Rd4 are designed to fall within a predetermined resistance range when heated to approximately 150°C. Therefore, the resistance values of thermistors Rd1 and Rd4 heated to approximately 300°C are sufficiently high to have a positive temperature coefficient of resistance. Therefore, the influence of thermistor Rd4 during period T1 and the influence of thermistor Rd1 during period T2 are reduced to a negligible level.

[0097] In the gas sensor 5 according to the fifth embodiment, as illustrated, the thermistor films 31 to 34 may be formed of a material having a positive temperature coefficient of resistance.

[0098] While the embodiments of the technology disclosed herein have been described above, the technology disclosed herein is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the invention, which are naturally also included in the scope of the technology disclosed herein.

[0099] For example, in the above-described embodiment, a thermistor is used as the temperature sensing element, but the present invention is not limited to this.

[0100] The technology disclosed herein includes the following structures, but is not limited thereto.

[0101] A gas sensor according to one aspect of the present disclosure includes: a first series circuit including first and second temperature-sensing elements connected in series; a second series circuit including third and fourth temperature-sensing elements connected in series; a first power supply circuit for applying a voltage to the first series circuit; a second power supply circuit for applying a voltage to the second series circuit; and a signal processing circuit for detecting a first detection voltage present at a first connection point connecting the first and second temperature-sensing elements during a first period when the first and third temperature-sensing elements are heated to a first temperature range and the second and fourth temperature-sensing elements are heated to a second temperature range, and detecting a second detection voltage present at a second connection point connecting the third and fourth temperature-sensing elements during a second period when the first and third temperature-sensing elements are heated to a second temperature range and the second and fourth temperature-sensing elements are heated to the first temperature range, wherein the signal processing circuit calculates the concentration of a target gas based on the first and second detection voltages. This arrangement allows the thermal histories of the first and second temperature-sensing elements to be aligned, and the thermal histories of the third and fourth temperature-sensing elements to be aligned, and allows the concentration of the target gas to be measured during both the first and second periods.

[0102] In the aforementioned gas sensor, the resistance values of the first and fourth temperature-sensing elements may be in the first resistance range when heated to the first temperature range, while the resistance values of the second and third temperature-sensing elements may be in the second resistance range when heated to the second temperature range. This allows detection of the concentration of non-combustible gases such as CO2 contained in the atmosphere. In this case, the first and second resistance regions may overlap, thereby achieving a wide dynamic range.

[0103] The gas sensor may further include a first heater for heating the first and third temperature sensing elements together, and a second heater for heating the second and fourth temperature sensing elements together. This allows the first and third temperature sensing elements to be heated to the same temperature, and the second and fourth temperature sensing elements to be heated to the same temperature.

[0104] In the above-mentioned gas sensor, the first temperature sensing element may be composed of a first portion of the first thermistor film heated by the first heater and a pair of first opposing electrodes opposing each other across the first portion of the first thermistor film; the second temperature sensing element may be composed of a first portion of the second thermistor film heated by the second heater and a pair of second opposing electrodes opposing each other across the first portion of the second thermistor film; the third temperature sensing element may be composed of a first portion of the third thermistor film heated by the first heater and a pair of third opposing electrodes opposing each other across the first portion of the third thermistor film; and the fourth temperature sensing element may be composed of a first portion of the fourth thermistor film heated by the second heater and a pair of fourth opposing electrodes opposing each other across the first portion of the fourth thermistor film. In this manner, the insulation between the first and third opposing electrodes is improved, and the insulation between the second and fourth opposing electrodes is improved.

[0105] In the above-described gas sensor, the first temperature sensing element may be composed of a first portion of the first thermistor film heated by the first heater and a pair of first opposing electrodes facing each other across the first portion of the first thermistor film; the second temperature sensing element may be composed of a first portion of the second thermistor film heated by the second heater and a pair of second opposing electrodes facing each other across the first portion of the second thermistor film; the third temperature sensing element may be composed of a second portion of the first thermistor film and a pair of third opposing electrodes facing each other across the second portion of the first thermistor film; and the fourth temperature sensing element may be composed of a second portion of the second thermistor film and a pair of fourth opposing electrodes facing each other across the second portion of the second thermistor film. This allows the first to fourth temperature sensing elements to be miniaturized.

[0106] In the aforementioned gas sensor, the first to fourth temperature sensing elements may have negative temperature coefficients of resistance, the temperature in the second temperature region may be higher than that in the first temperature region, the relative length of the first opposing electrodes may be longer than that of the second opposing electrodes, and the relative length of the fourth opposing electrodes may be longer than that of the third opposing electrodes. In this manner, the resistance value of the first temperature sensing element can be lower than that of the second temperature sensing element, and the resistance value of the fourth temperature sensing element can be lower than that of the third temperature sensing element.

[0107] In the aforementioned gas sensor, the first to fourth temperature sensing elements may have a negative temperature coefficient of resistance, the temperature in the second temperature region may be higher than the temperature in the first temperature region, the inter-electrode distance between the first opposing electrodes may be shorter than the inter-electrode distance between the second opposing electrodes, and the inter-electrode distance between the fourth opposing electrodes may be shorter than the inter-electrode distance between the third opposing electrodes. In this manner, the resistance value of the first temperature sensing element can be lower than the resistance value of the second temperature sensing element, and the resistance value of the fourth temperature sensing element can be lower than the resistance value of the third temperature sensing element.

[0108] In the aforementioned gas sensor, the first to fourth temperature sensing elements may have positive temperature coefficients of resistance, the temperature in the second temperature region may be higher than that in the first temperature region, the relative length of the second opposing electrodes may be longer than that of the first opposing electrodes, and the relative length of the third opposing electrodes may be longer than that of the fourth opposing electrodes. Thus, the resistance value of the second temperature sensing element can be lower than that of the first temperature sensing element, and the resistance value of the third temperature sensing element can be lower than that of the fourth temperature sensing element.

[0109] In the aforementioned gas sensor, the first to fourth temperature sensing elements may have positive temperature coefficients of resistance, the temperature in the second temperature region may be higher than the temperature in the first temperature region, the inter-electrode distance between the second opposing electrodes may be shorter than the inter-electrode distance between the first opposing electrodes, and the inter-electrode distance between the third opposing electrodes may be shorter than the inter-electrode distance between the fourth opposing electrodes. In this manner, the resistance value of the second temperature sensing element can be lower than the resistance value of the first temperature sensing element, and the resistance value of the third temperature sensing element can be lower than the resistance value of the fourth temperature sensing element.

[0110] In the above-described gas sensor, the first to fourth temperature sensing elements may have a negative temperature coefficient of resistance, the temperature in the second temperature region may be higher than that in the first temperature region, the first power supply circuit may include a first switch connected between a first power supply line or ground line supplying a first power supply voltage and the first temperature sensing element, and the second power supply circuit may include a second switch connected between a second power supply line or ground line supplying a second power supply voltage and the fourth temperature sensing element. This allows the first temperature sensing element to be disconnected from the first power supply line or ground line, and the fourth temperature sensing element to be disconnected from the second power supply line or ground line. In this case, the first connection point and the second connection point may be short-circuited, and the signal processing circuit may turn on the first switch and turn off the second switch during a first period, and turn off the first switch and turn on the second switch during a second period. This allows the fourth temperature sensing element to be disconnected from the second power supply line or ground line during the first period, and the first temperature sensing element to be disconnected from the first power supply line or ground line during the second period.

[0111] In the above-described gas sensor, the first to fourth temperature-sensing elements may have a positive temperature coefficient of resistance, the temperature in the second temperature region may be higher than that in the first temperature region, the first power supply circuit may include a first switch connected between a first power supply line or ground line supplying the first power supply voltage and the second temperature-sensing element, and the second power supply circuit may include a second switch connected between a second power supply line or ground line supplying the second power supply voltage and the third temperature-sensing element. This allows the second temperature-sensing element to be disconnected from the first power supply line or ground line, and the third temperature-sensing element to be disconnected from the second power supply line or ground line. In this case, the first connection point and the second connection point may be short-circuited, and the signal processing circuit may turn on the first switch and turn off the second switch during a first period, and turn off the first switch and turn on the second switch during a second period. This allows the third temperature-sensing element to be disconnected from the second power supply line or ground line during the first period, and the second temperature-sensing element to be disconnected from the first power supply line or ground line during the second period.

[0112] In the above-described gas sensor, the first power supply circuit may include a first switch connected between one of a first power supply line and a ground line supplying a first power supply voltage and the first temperature sensing element, and a third switch connected between the other of the first power supply line and the ground line and the second temperature sensing element. The second power supply circuit may include a second switch connected between one of a second power supply line and the ground line supplying a second power supply voltage and the fourth temperature sensing element, and a fourth switch connected between the other of the second power supply line and the ground line and the third temperature sensing element. This allows the first temperature sensing element to be disconnected from one of the first power supply line and the ground line, the second temperature sensing element to be disconnected from the other of the first power supply line and the ground line, the third temperature sensing element to be disconnected from the other of the second power supply line and the ground line, and the fourth temperature sensing element to be disconnected from one of the second power supply line and the ground line. In this case, the first connection point and the second connection point may be short-circuited, and the signal processing circuit may turn on the first and third switches and turn off the second and fourth switches during a first period, and turn off the first and third switches and turn on the second and fourth switches during a second period. As a result, the second connection point is in an open state during the first period, and the first connection point is in an open state during the second period.

[0113] In the aforementioned gas sensor, the signal processing circuit may calculate a moving average of the concentration of the target gas obtained based on the first detection voltage and the concentration of the target gas obtained based on the second detection voltage. This allows the offset to be eliminated even when there is an offset between the first detection voltage and the second detection voltage.

Claims

1. A gas sensor, wherein: have: A first series circuit comprising first and second temperature sensing elements connected in series; a second series circuit comprising a third and a fourth temperature sensing element connected in series; a first power supply circuit for applying a voltage to the first series circuit; a second power supply circuit that applies a voltage to the second series circuit; and a signal processing circuit that detects a first detection voltage present at a first connection point connecting the first temperature sensing element and the second temperature sensing element during a first period in which the first and third temperature sensing elements are heated to a first temperature range and the second and fourth temperature sensing elements are heated to a second temperature range, and detects a second detection voltage present at a second connection point connecting the third temperature sensing element and the fourth temperature sensing element during a second period in which the first and third temperature sensing elements are heated to the second temperature range and the second and fourth temperature sensing elements are heated to the first temperature range, The signal processing circuit calculates the concentration of the detection target gas based on the first and second detection voltages.

2. The gas sensor according to claim 1, wherein When the first and fourth temperature sensing elements are heated to the first temperature range, the resistance values thereof are in the first resistance range. The resistance values of the second and third temperature sensing elements fall within a second resistance range when heated to the second temperature range.

3. The gas sensor according to claim 2, wherein: The first resistance region and the second resistance region have an overlap.

4. The gas sensor according to claim 1, wherein Also features: a first heater that heats both the first and third temperature sensing elements; and The second heater heats the second and fourth temperature sensing elements together.

5. The gas sensor according to claim 4, wherein The first temperature sensing element is composed of a first portion of a first thermistor film heated by the first heater and a pair of first opposing electrodes facing each other with the first portion of the first thermistor film interposed therebetween. The second temperature sensing element is composed of a first portion of a second thermistor film heated by the second heater and a pair of second opposing electrodes facing each other with the first portion of the second thermistor film interposed therebetween. The third temperature sensing element is composed of a first portion of a third thermistor film heated by the first heater and a pair of third opposing electrodes facing each other with the first portion of the third thermistor film interposed therebetween. The fourth temperature sensing element includes a first portion of a fourth thermistor film heated by the second heater and a pair of fourth opposing electrodes facing each other with the first portion of the fourth thermistor film interposed therebetween.

6. The gas sensor according to claim 4, wherein The first temperature sensing element is composed of a first portion of a first thermistor film heated by the first heater and a pair of first opposing electrodes facing each other with the first portion of the first thermistor film interposed therebetween. The second temperature sensing element is composed of a first portion of a second thermistor film heated by the second heater and a pair of second opposing electrodes facing each other with the first portion of the second thermistor film interposed therebetween. The third temperature sensing element is composed of the second portion of the first thermistor film and a pair of third opposing electrodes facing each other with the second portion of the first thermistor film interposed therebetween. The fourth temperature sensing element includes a second portion of the second thermistor film and a pair of fourth opposing electrodes facing each other with the second portion of the second thermistor film interposed therebetween.

7. The gas sensor according to claim 5 or 6, wherein: The first to fourth temperature sensing elements have negative temperature coefficients of resistance. The temperature of the second temperature region is higher than that of the first temperature region. The relative length of the first opposing electrodes is longer than the relative length of the second opposing electrodes. The relative length of the fourth opposing electrodes is longer than the relative length of the third opposing electrodes.

8. The gas sensor according to claim 5 or 6, wherein: The first to fourth temperature sensing elements have negative temperature coefficients of resistance. The temperature of the second temperature region is higher than that of the first temperature region. The inter-electrode distance of the first opposing electrodes is shorter than the inter-electrode distance of the second opposing electrodes, An inter-electrode distance of the fourth opposing electrodes is shorter than an inter-electrode distance of the third opposing electrodes.

9. The gas sensor according to claim 5 or 6, wherein: The first to fourth temperature sensing elements have positive temperature coefficients of resistance. The temperature of the second temperature region is higher than that of the first temperature region. The relative length of the second opposing electrodes is longer than the relative length of the first opposing electrodes. The relative length of the third opposing electrodes is longer than the relative length of the fourth opposing electrodes.

10. The gas sensor according to claim 5 or 6, wherein: The first to fourth temperature sensing elements have positive temperature coefficients of resistance. The temperature of the second temperature region is higher than that of the first temperature region. The inter-electrode distance of the second opposing electrodes is shorter than the inter-electrode distance of the first opposing electrodes, An inter-electrode distance of the third opposing electrodes is shorter than an inter-electrode distance of the fourth opposing electrodes.

11. The gas sensor according to claim 1, wherein The first to fourth temperature sensing elements have negative temperature coefficients of resistance. The temperature of the second temperature region is higher than that of the first temperature region. The first power supply circuit includes a first switch connected between a first power supply line or a ground line supplying a first power supply voltage and the first temperature sensing element. The second power supply circuit includes a second switch connected between a second power supply line or a ground line that supplies a second power supply voltage and the fourth temperature sensing element.

12. The gas sensor according to claim 1, wherein The first to fourth temperature sensing elements have positive temperature coefficients of resistance. The temperature of the second temperature region is higher than that of the first temperature region. The first power supply circuit includes a first switch connected between a first power supply line or a ground line supplying a first power supply voltage and the second temperature sensing element. The second power supply circuit includes a second switch connected between a second power supply line or a ground line that supplies a second power supply voltage and the third temperature sensing element.

13. The gas sensor according to claim 11 or 12, wherein: The first connection point and the second connection point are short-circuited, The signal processing circuit turns on the first switch and turns off the second switch during the first period, and turns off the first switch and turns on the second switch during the second period.

14. The gas sensor according to claim 1, wherein The first power supply circuit includes a first switch connected between the first temperature sensing element and one of a first power supply line and a ground line for supplying a first power supply voltage, and a third switch connected between the second temperature sensing element and the other of the first power supply line and the ground line. The second power supply circuit includes a second switch connected between a second power supply line supplying a second power supply voltage and one of the ground line and the fourth temperature sensing element, and a fourth switch connected between the other of the second power supply line and the ground line and the third temperature sensing element.

15. The gas sensor according to claim 14, wherein The first connection point and the second connection point are short-circuited, The signal processing circuit turns on the first and third switches and turns off the second and fourth switches during the first period, and turns off the first and third switches and turns on the second and fourth switches during the second period.

16. The gas sensor according to claim 1, wherein The signal processing circuit calculates a moving average of the concentration of the detection target gas obtained based on the first detection voltage and the concentration of the detection target gas obtained based on the second detection voltage.

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