Gas sensor

Abstract

To provide a gas sensor including a thermo-sensitive element, having increased period for detecting density of a detection target gas.SOLUTION: In a period T1 for heating thermistors Rd1, Rd3 to a first temperature region and thermistors Rd2, Rd4 to a second temperature region, a gas sensor 1 detects detection voltage generated at a junction point N1 between the thermistor Rd1 and the thermistor Rd2, and detects detection voltage generated at a junction point N2 between the thermistor Rd3 and the thermistor Rd4 in a period T2 for heating the thermistors Rd1, Rd3 to the second temperature region and the thermistors Rd2, Rd4 to the first temperature region.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a gas sensor, and more particularly to a gas sensor equipped with a temperature-sensitive element such as a thermistor. [Background technology]

[0002] Patent Document 1 discloses a gas sensor equipped with two thermistors connected in series. In the gas sensor described in Patent Document 1, one thermistor is heated to a first temperature range and the other to a second temperature range during a first period, and one thermistor is heated to the second temperature range and the other to the first temperature range during a second period, thereby matching the difference in thermal history between the two thermistors. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2020 / 031517 Summary of the Invention [Problem to be solved by the invention]

[0004] However, the gas sensor described in Patent Document 1 cannot detect the concentration of the target gas during the second period.

[0005] The present disclosure describes a technique for increasing the period during which a concentration of a target gas can be detected in a gas sensor equipped with a temperature-sensitive element such as a thermistor. [Means for solving the problem]

[0006] A gas sensor according to one aspect of the present disclosure comprises a first series circuit including first and second thermosensitive elements connected in series, a second series circuit including third and fourth thermosensitive elements connected in series, a first power supply circuit that applies 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 that appears at a first connection point where the first thermosensitive element and the second thermosensitive element are connected during a first period in which the first and third thermosensitive elements are heated to a first temperature range and the second and fourth thermosensitive elements are heated to a second temperature range, and that detects a second detection voltage that appears at a second connection point where the third thermosensitive element and the fourth thermosensitive element are connected during a second period in which the first and third thermosensitive elements are heated to a second temperature range and the second and fourth thermosensitive elements are heated to the first temperature range, and the signal processing circuit calculates the concentration of a target gas based on the first and second detection voltages. [Effects of the Invention]

[0007] According to the present disclosure, a technology is provided for increasing the period during which the concentration of a target gas can be detected in a gas sensor equipped with a temperature-sensitive element such as a thermistor. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a circuit diagram showing a configuration of a gas sensor 1 according to a first embodiment of the technique disclosed herein. [Figure 2] FIG. 2 is a schematic plan view for explaining the device structure of the sensor units S1 and S2, showing the portion relating to the heater resistors MH1 and MH2. [Figure 3] FIG. 3 is a schematic plan view for explaining the device structure of the sensor units S1 and S2, showing the parts related to thermistors Rd1 to Rd4. [Figure 4] FIG. 4 is a schematic plan view for explaining the device structure of the sensor units S1 and S2, and shows a state in which FIG. 2 and FIG. 3 are superimposed. [Figure 5] FIG. 5 is a schematic plan view for explaining a modified example of the device structure of the sensor units S1 and S2, showing the portion relating to thermistors Rd1 to Rd4. [Figure 6] FIG. 6 is a flowchart illustrating the operation of the gas sensor 1. [Figure 7] FIG. 7 is a timing chart for explaining the operation of the gas sensor 1. As shown in FIG. [Figure 8] FIG. 8 is a flowchart for explaining the operation of the modified example of the gas sensor 1. [Figure 9] Figure 9(a) is a graph showing the change in the measurement result signal OUT obtained when there is an offset between the detection voltage Vgas1 appearing at connection point N1 and the detection voltage Vgas1 appearing at connection point N2, and Figure 9(b) is a graph showing the change in the average value of the measurement result signal OUT obtained in a measurement at a certain point in time and the measurement result signal OUT obtained in the measurement immediately before that. [Figure 10] FIG. 10 is a circuit diagram showing a configuration of a gas sensor 2 according to a second embodiment of the technique disclosed herein. [Figure 11] FIG. 11 is a circuit diagram showing a configuration of a gas sensor 3 according to a third embodiment of the technique disclosed herein. [Figure 12] FIG. 12 is a circuit diagram showing the configuration of a gas sensor 4 according to a fourth embodiment of the technique disclosed herein. [Figure 13] FIG. 13 is a circuit diagram showing a configuration of a gas sensor 5 according to a fifth embodiment of the technique disclosed herein. [Figure 14] FIG. 14 is a schematic plan view for explaining the device structure of the sensor sections S1 and S2 in the gas sensor 5, showing the parts related to thermistors Rd1 to Rd4. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the technology according to the present disclosure will be described in detail with reference to the accompanying drawings.

[0010] First Embodiment FIG. 1 is a circuit diagram showing a configuration of a gas sensor 1 according to a first embodiment of the technique disclosed herein.

[0011] 1, the gas sensor 1 according to the first embodiment includes sensor units S1 and S2 and a signal processing circuit 10. Although not particularly limited, the gas sensor 1 according to this embodiment is a thermal conduction type gas sensor for detecting the concentration of CO2 gas in the atmosphere.

[0012] The sensor section S1 includes thermistors Rd1 and Rd3 and a heater resistor MH1. The sensor section S2 includes thermistors Rd2 and Rd4 and a heater resistor MH2. The thermistors Rd1 to Rd4 all have negative temperature coefficients of resistance. Thermistors Rd1 and Rd2 are connected in series, thereby forming a first series circuit. Thermistors Rd3 and Rd4 are connected in series, thereby forming 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. In the example shown in FIG. 1, the connection points N1 and N2 are short-circuited.

[0013] The other end of thermistor Rd1 is connected via switch SW1 to a line through which power supply voltage Vcc1 is supplied. The other end of thermistor Rd2 is connected via switch SW3 to a ground line. The other end of thermistor Rd3 is connected via switch SW4 to a ground line. The other end of thermistor Rd4 is connected via switch SW2 to a line through which power supply voltage Vcc2 is supplied. Power supply voltages Vcc1 and Vcc2 may be at the same level. Switches SW1 and SW3, the line through which power supply voltage Vcc1 is supplied, and the ground line constitute a first power supply circuit that applies a voltage to a first series circuit consisting of thermistors Rd1 and Rd2. Switches SW2 and SW4, the line through which power supply voltage Vcc2 is supplied, and the ground line constitute a second power supply circuit that applies a voltage to a second series circuit consisting of thermistors Rd3 and Rd4. In the first series circuit consisting of thermistors Rd1 and Rd2, the connection relationship between the power supply voltage Vcc1 and ground may be reversed. That is, the other end of thermistor Rd1 may be connected to the ground wiring via switch SW1, and the other end of thermistor Rd2 may be connected to the wiring that supplies power supply voltage Vcc1 via switch SW3. Similarly, in the second series circuit consisting of thermistors Rd3 and Rd4, the connection relationship between the power supply voltage Vcc2 and ground may be reversed. That is, the other end of thermistor Rd3 may be connected to the wiring that supplies power supply voltage Vcc2 via switch SW4, and the other end of thermistor Rd4 may be connected to the ground wiring via switch SW2.

[0014] The resistance values ​​of the thermistors Rd1 and Rd4, which are temperature-sensing elements for detection, are designed to be in a first resistance range when heated to a first temperature range (e.g., approximately 150°C). The resistance values ​​of the thermistors Rd2 and Rd3, which are reference temperature-sensing elements, are designed to be in a second resistance range when heated to a second temperature range (e.g., approximately 300°C). In this embodiment, the second temperature range is higher than the first temperature range. The first and second resistance ranges may partially overlap or may coincide. When the first and second resistance ranges overlap or coincide, the level of the detection voltage Vgas1 appearing at the nodes N1 and N2 can be set to be near Vcc1 / 2 or Vcc2 / 2, thereby achieving a wide dynamic range. The detection voltage Vgas1 appearing at the nodes N1 and N2 is supplied to the signal processing circuit 10.

[0015] When the thermistors Rd1 and Rd4, which are temperature-sensing elements used for detection, are heated to around 150°C and CO2 gas is present in the measurement atmosphere, the heat dissipation characteristics of the thermistors Rd1 and Rd4 change depending on the concentration of CO2 gas. This change manifests itself as a change in the temperature of the thermistors Rd1 and Rd4, i.e., a change in the resistance of the thermistors Rd1 and Rd4. Specifically, because CO2 gas has a lower heat dissipation rate than air, the higher the CO2 gas concentration, the higher the temperature of the thermistors Rd1 and Rd4. For example, if the thermistors Rd1 and Rd4 are heated to 150°C when the CO2 gas concentration in the measurement atmosphere is the same as the average concentration in the atmosphere, the temperature of the thermistors Rd1 and Rd4 will be higher than 150°C if the CO2 gas concentration in the measurement atmosphere is higher than the average concentration in the atmosphere. As a result, the resistance of the thermistors Rd1 and Rd4 will be lower than when the CO2 gas concentration in the measurement atmosphere is the average concentration in the atmosphere.

[0016] On the other hand, even if the reference thermistors Rd2 and Rd3 are heated to around 300°C and CO2 gas is present in the measurement atmosphere, the heat dissipation characteristics of the thermistors Rd2 and Rd3 change very little with CO2 gas concentration, and the temperature of the thermistors Rd2 and Rd3 also changes very little. Therefore, the change in resistance of thermistors Rd2 and Rd3 with CO2 gas concentration when heated to around 300°C is significantly smaller than the change in resistance of thermistors Rd1 and Rd4 when heated to around 150°C. The change in resistance of thermistors Rd2 and Rd3 with CO2 gas concentration when heated to around 300°C is not necessarily significant. As a result, when thermistor Rd1 is heated to around 150°C and thermistor Rd2 is heated to around 300°C, a detection voltage Vgas1 appears at node N1 that corresponds to the CO2 gas concentration in the measurement atmosphere. Similarly, when thermistor Rd4 is heated to around 150°C and thermistor Rd3 is heated to around 300°C, a detection voltage Vgas1 corresponding to the concentration of CO2 gas in the measurement atmosphere appears at node N2. On the other hand, even if the measurement atmosphere contains another gas whose heat dissipation characteristics are similar between when thermistors Rd1 to Rd4 are heated to around 150°C and when they are heated to around 300°C, the concentration of that gas has almost no effect on the detection voltage Vgas1. This makes it possible to selectively detect the concentration of CO2 gas.

[0017] The signal processing circuit 10 includes an amplifier 11, an AD converter (ADC) 12, a DA converter (DAC) 13, and a control circuit 14. The amplifier 11 generates an amplified detection voltage Vgas2 by comparing a detection voltage Vgas1 with a reference voltage Vref. The amplifier 11 is, for example, a differential amplifier, and generates and outputs the detection voltage Vgas2 by amplifying the difference between the detection voltage Vgas1 and the reference voltage Vref. 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 supplied to the control circuit 14.

[0018] The control circuit 14 supplies digital values ​​of various control parameters to the DA converter 13. The DA converter 13 converts the digital values ​​of the various control parameters into analog values ​​to generate heater voltages Vmh1 and Vmh2 and a reference voltage Vref. The heater voltage Vmh1 is applied to the heater resistor MH1, thereby heating the thermistors Rd1 and Rd3 in common. The heater voltage Vmh2 is applied to the heater resistor MH2, thereby heating the thermistors Rd2 and Rd4 in common. The reference voltage Vref is also supplied to the amplifier 11.

[0019] Figures 2 to 4 are schematic plan views for explaining the device structure of the sensor sections S1 and S2, where Figure 2 shows the portion relating to heater resistors MH1 and MH2, Figure 3 shows the portion relating to thermistors Rd1 to Rd4, and Figure 4 is a diagram in which Figures 2 and 3 are superimposed.

[0020] 2 to 4, the sensor units S1 and S2 each include a substrate 20, an insulating film 21 provided on the surface of the substrate 20, heater resistors MH1 and MH2 provided on the insulating film 21, thermistor films 31 and 33 provided at positions overlapping with the heater resistor MH1, thermistor films 32 and 34 provided at positions overlapping with the heater resistor MH2, a pair of counter electrodes 41 and 42 in contact with thermistor film 31, a pair of counter electrodes 43 and 44 in contact with thermistor film 32, a pair of counter electrodes 45 and 46 in contact with thermistor film 33, and a pair of counter electrodes 47 and 48 in contact with thermistor film 34. A first portion of the thermistor film 31 located between the pair of counter electrodes 41 and 42 and the counter electrodes 41 and 42 constitutes a thermistor Rd1. A first portion of the thermistor film 32 located between the pair of counter electrodes 43 and 44 and the counter electrodes 43 and 44 constitutes a thermistor Rd2. The thermistor Rd3 is formed by a first portion of the thermistor film 33 located between the pair of counter electrodes 45, 46, and the counter electrodes 45, 46. The thermistor Rd4 is formed by a first portion of the thermistor film 34 located between the pair of counter electrodes 47, 48, and the counter electrodes 47, 48.

[0021] The substrate 20 is not particularly limited as long as it has a suitable material for microfabrication such as etching and has adequate mechanical strength, and may be made of a silicon single crystal substrate, a sapphire single crystal substrate, a ceramic substrate, a quartz substrate, a glass substrate, or the like. The substrate 20 and the insulating film 21 are removed at the cutouts 20A and 20B, forming hollow spaces. The substrate 20 is removed in the areas surrounded by the cutouts 20A and 20B, and the heater resistors MH1 and MH2 and thermistors Rd1 to Rd4 are supported by the thin insulating film 21. This membrane structure can suppress the conduction of heat from the heater resistors MH1 and MH2 to the substrate 20.

[0022] The heater resistors MH1 and MH2 are made of a conductive material, and may be made of a metal material having a relatively high melting point, such as molybdenum (Mo), platinum (Pt), gold (Au), tungsten (W), tantalum (Ta), palladium (Pd), iridium (Ir), nickel (Ni), chromium (Cr), or an alloy containing two or more of these. As shown in FIG. 2, one end of the heater resistor MH1 is connected to a terminal electrode 51, and the other end of the heater resistor MH2 is connected to a terminal electrode 53. Furthermore, one end of the heater resistor MH2 is connected to a terminal electrode 52, and the other end of the heater resistor MH2 is connected to the terminal electrode 53. The heater resistors MH1 and MH2 may have a serpentine shape.

[0023] The thermistor films 31-34 are made of a material with a negative temperature coefficient of resistance, such as a composite metal oxide, amorphous silicon, polysilicon, or germanium. The counter electrodes 41 and 42 in contact with the thermistor film 31 are connected to terminal electrodes 54 and 56, respectively. As a result, the terminal electrodes 54 and 56 are connected via the thermistor film 31 located between the counter electrodes 41 and 42. The counter electrodes 43 and 44 in contact with the thermistor film 32 are connected to terminal electrodes 56 and 55, respectively. As a result, the terminal electrodes 55 and 56 are connected via the thermistor film 32 located between the counter electrodes 43 and 44. The counter electrodes 45 and 46 in contact with the thermistor film 33 are connected to terminal electrodes 57 and 59, respectively. As a result, the terminal electrodes 57 and 59 are connected via the thermistor film 33 located between the counter electrodes 45 and 46. The counter electrodes 47 and 48 in contact with the thermistor film 34 are connected to the terminal electrodes 59 and 58, respectively. As a result, the terminal electrodes 58 and 59 are connected via the thermistor film 34 located between the counter electrodes 47 and 48.

[0024] Terminal electrode 54 is connected to a wiring line to which power supply voltage Vcc1 is supplied via switch SW1. Terminal electrode 55 is connected to a ground wiring line via switch SW3. Terminal electrode 58 is connected to a wiring line to which power supply voltage Vcc2 is supplied via switch SW2. Terminal electrode 57 is connected to a ground wiring line via switch SW4. Switches SW1 to SW4 may be part of signal processing circuit 10. Terminal electrode 56 forms connection point N1. Terminal electrode 59 forms connection point N2.

[0025] 3, the counter electrodes 41, 42 and the counter electrodes 47, 48 have comb-like interdigitated shapes, whereas the counter electrodes 43, 44 and the counter electrodes 45, 46 have linear opposing shapes. As a result, the opposing length between the counter electrodes 41, 42 and the counter electrodes 47, 48 is longer than the opposing length between the counter electrodes 43, 44 and the counter electrodes 45, 46. Furthermore, the interelectrode distance (opposing width) between the counter electrodes 41, 42 and the counter electrodes 47, 48 is shorter than the interelectrode distance (opposing width) between the counter electrodes 43, 44 and the counter electrodes 45, 46. As a result, when the resistivities of the thermistor films 31 to 34 are approximately the same and the thermistor films 31 to 34 are heated to approximately the same temperature, the resistance value of the thermistor film 31 located between the counter electrodes 41 and 42 and the resistance value of the thermistor film 34 located between the counter electrodes 47 and 48 will be lower than the resistance value of the thermistor film 32 located between the counter electrodes 43 and 44 and the resistance value of the thermistor film 33 located between the counter electrodes 45 and 46. This is because, as mentioned above, the resistance values ​​of the thermistors Rd1 and Rd4, which are the temperature-sensing elements used for detection, are designed to be in the first resistance range when heated to approximately 150°C, whereas the resistance values ​​of the thermistors Rd2 and Rd3, which are the reference temperature-sensing elements, are designed to be in the second resistance range when heated to approximately 300°C.In order to overlap or match the first resistance range and the second resistance range, the resistance values ​​of thermistors Rd1 and Rd4 must be designed to be lower than the resistance values ​​of thermistors Rd2 and Rd3 at the same temperature.

[0026] In the example shown in Figure 3, thermistor film 31 and thermistor film 33 are separated, and thermistor film 32 and thermistor film 34 are separated, but as in the example shown in Figure 5, it is also possible to use a common thermistor film 35 for opposing electrodes 41, 42, 45, and 46, and a common thermistor film 36 for opposing electrodes 43, 44, 47, and 48. In this case, the first portion of the thermistor film 35 located between the pair of opposing electrodes 41, 42 and the opposing electrodes 41, 42 constitute thermistor Rd1, the first portion of the thermistor film 36 located between the pair of opposing electrodes 43, 44 and the opposing electrodes 43, 44 constitute thermistor Rd2, the second portion of the thermistor film 35 located between the pair of opposing electrodes 45, 46 and the opposing electrodes 45, 46 constitute thermistor Rd3, and the second portion of the thermistor film 36 located between the pair of opposing electrodes 47, 48 and the opposing electrodes 47, 48 constitute thermistor Rd4.

[0027] Next, the operation of the gas sensor 1 according to the first embodiment will be described.

[0028] Fig. 6 is a flowchart illustrating the operation of the gas sensor 1. Fig. 7 is a timing chart illustrating the operation of the gas sensor 1.

[0029] First, the signal processing circuit 10 included in the gas sensor 1 acquires 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 offsets in the heater voltages Vmh1 and Vmh2 and the reference voltage Vref caused by the temperature of the measurement atmosphere.

[0030] Next, the signal processing circuit 10 performs an operation 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, the 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, when the CO2 gas concentration in the measurement atmosphere is the same as the average concentration in the air, the heater voltages Vmh1 and Vmh2 are set so that 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 node N1. The detected voltage Vgas1 is input to the signal processing circuit 10 (step 104), and a control circuit 14 included in the signal processing circuit 10 calculates a measurement result signal OUT indicating the CO2 gas concentration and outputs it to the outside (step 105). During the period T1, the thermistors Rd3 and Rd4 are also heated. During the period T1, the switches SW2 and SW4 are off, so the thermistors Rd3 and Rd4 do not affect the detected voltage Vgas1.

[0031] Next, the signal processing circuit 10 performs an operation 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, when the CO2 gas concentration in the measurement atmosphere is the same as the average concentration in the air, the heater voltages Vmh1 and Vmh2 are set so that 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 node N2. The detected voltage Vgas1 is input to the signal processing circuit 10 (step 108), and a control circuit 14 included in the signal processing circuit 10 calculates a measurement result signal OUT indicating the CO2 gas concentration and outputs it to the outside (step 109). During the period T2, the thermistors Rd1 and Rd2 are also heated. During the period T2, the switches SW1 and SW3 are off, so the thermistors Rd1 and Rd2 do not affect the detected voltage Vgas1.

[0032] By repeating the above steps 102 to 109 until the measurement is completed, it becomes possible to periodically acquire the measurement result signal OUT (step 110).

[0033] As described above, the gas sensor 1 according to the first embodiment alternates between a period T1 in which the thermistors Rd1 and Rd3 are heated to approximately 150°C and the thermistors Rd2 and Rd4 are heated to approximately 300°C and a 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 makes it possible to obtain a measurement result signal OUT in both periods T1 and T2 while matching the thermal histories of thermistors Rd1 and Rd2 and thermistors Rd3 and Rd4. However, it is not essential to alternate between periods T1 and T2; the operation in period T1 may be performed multiple times, and then the operation in period T2 may be performed multiple times.

[0034] 8 is a flowchart for explaining the operation of the modified example of the gas sensor 1. The same operations as those explained with reference to FIG. 6 are denoted by the same reference numerals.

[0035] First, after acquiring the temperature signal T (step 100) and calculating the correction value (step 101), the signal processing circuit 10 performs the operation for the first time in the period T1. The operation for the first time in the period T1 (steps 202 to 205) is basically the same as the above-mentioned steps 102 to 105, but in step 205, only the calculation of the measurement result signal OUT is performed, and the measurement result signal OUT is not output to the outside.

[0036] Next, the signal processing circuit 10 performs an operation in a period T2. The operation in the period T2 is basically the same as the above-mentioned steps 106 to 109, but in step 109 where the measurement result signal OUT is output, the measurement result signal OUT calculated in step 205 and the average value of the measurement result signal OUT calculated in step 109 are output to the outside.

[0037] Next, the signal processing circuit 10 performs an operation in a period T1. The operation in the period T1 is basically the same as the above-mentioned steps 102 to 105, but in step 105 where the measurement result signal OUT is output, the measurement result signal OUT calculated in step 109 and the average value of the measurement result signal OUT calculated in step 105 are output to the outside.

[0038] 8, the average value of the measurement result signal OUT obtained in a measurement at a certain point in time and the measurement result signal OUT obtained in the measurement immediately before that is output to the outside, so even if there is an offset between the detection voltage Vgas1 appearing at connection point N1 and the detection voltage Vgas1 appearing at connection point N2, as shown in Figure 9(a), the offset will not appear in the measurement result signal OUT that is actually output, as shown in Figure 9(b). Note that in the above example, the moving average of two measurement result signals OUT is output, but it is also possible to calculate and output a moving average of more measurement result signals OUT (for example, the measurement result signal OUT obtained in the current measurement and the measurement result signals OUT obtained in the immediately previous multiple measurements).

[0039] <Second embodiment> FIG. 10 is a circuit diagram showing a configuration of a gas sensor 2 according to a second embodiment of the technique disclosed herein.

[0040] 10, the gas sensor 2 according to the second embodiment differs from the gas sensor 1 according to the first embodiment in that the switches SW3 and SW4 are eliminated and the other ends of the thermistors Rd2 and Rd3 are directly connected to the ground wiring. Since the other basic configuration is the same as that of the gas sensor 1 according to the first embodiment, the same elements are denoted by the same reference numerals and redundant description will be omitted.

[0041] The operation of the gas sensor 2 according to the second embodiment is the same as that of the gas sensor 1 according to the first embodiment, except that the switches SW3 and SW4 are not controlled. In the gas sensor 2 according to the second embodiment, since the switches SW3 and SW4 are not present, the connection point N2 is connected to the ground wiring via the reference thermistor Rd3 during period T1, and the connection point N1 is connected to the ground wiring via the reference thermistor Rd2 during period T2. However, although the thermistor Rd3 is heated to approximately 150°C during period T1 and the thermistor Rd2 is heated to approximately 150°C during period T2, the resistances of the reference thermistors Rd2 and Rd3 are designed to be within a predetermined resistance range when heated to approximately 300°C. Therefore, the resistances of the thermistors Rd2 and Rd3 heated to approximately 150°C are sufficiently high due to their negative temperature coefficients of resistance. Therefore, the influence of the thermistor Rd3 during period T1 and the thermistor Rd2 during period T2 are negligible.

[0042] As described above, the gas sensor 2 according to the second embodiment does not include the switches SW3 and SW4, and therefore the circuit scale can be further simplified.

[0043] <Third embodiment> FIG. 11 is a circuit diagram showing a configuration of a gas sensor 3 according to a third embodiment of the technique disclosed herein.

[0044] 11 , the gas sensor 3 according to the third embodiment does not include the switches SW1 to SW4. Therefore, the other end of the thermistor Rd1 is directly connected to the wiring to which the power supply voltage Vcc1 is supplied, the other ends of the thermistors Rd2 and Rd3 are both directly connected to the ground wiring, and the other end of the thermistor Rd4 is directly connected to the wiring to which the power supply voltage Vcc2 is supplied. The gas sensor 3 according to the third embodiment also includes a switch SW5 that connects one of the connection points N1 and N2 to the amplifier 11. The switch SW5 is controlled by the signal processing circuit 10. When the connection point N1 is selected, the detection voltage Vgas1a appearing at the connection point N1 is supplied to the amplifier 11 as the detection voltage Vgas1. When the connection point N2 is selected, the detection voltage Vgas1b appearing at the connection point N2 is supplied to the amplifier 11 as the detection voltage Vgas1. The signal processing circuit 10 controls the switch SW5 to select the connection point N1 during the period T1, and controls the switch SW5 to select the connection point N2 during the period T2. Since the other basic configurations are the same as those of the gas sensor 1 according to the first embodiment, the same elements are denoted by the same reference numerals and redundant explanations will be omitted.

[0045] As described above, the gas sensor 3 according to the third embodiment is exclusively connected to the amplifier 11 via the switch SW5 without short-circuiting the connection points N1 and N2, and 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.

[0046] <Fourth embodiment> FIG. 12 is a circuit diagram showing the configuration of a gas sensor 4 according to a fourth embodiment of the technique disclosed herein.

[0047] As shown in FIG. 12, in the gas sensor 4 according to the fourth embodiment, a detection voltage Vgas1a appearing at a node N1 is supplied to an amplifier 11a, and a detection voltage Vgas1b appearing at a node 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 converts the detection voltages Vgas2a and Vgas2b into digital values ​​and supplies them to a control circuit 14. The other basic configuration is the same as that of the gas sensor 3 according to the third embodiment, so the same elements are designated by the same reference numerals and redundant description will be omitted.

[0048] In this way, in the gas sensor 4 according to the fourth embodiment, the connection points N1 and N2 are not short-circuited but are connected to different amplifiers 11a and 11b, respectively, so that 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.

[0049] <Fifth embodiment> FIG. 13 is a circuit diagram showing a configuration of a gas sensor 5 according to a fifth embodiment of the technique disclosed herein.

[0050] 13, the gas sensor 5 according to the fifth embodiment differs from the gas sensor 1 according to the first embodiment in that switches SW1 and SW2 are omitted, the other end of thermistor Rd1 is directly connected to a wiring that supplies power supply voltage Vcc1, the other end of thermistor Rd4 is directly connected to a wiring that supplies power supply voltage Vcc2, and the thermistor films 31-34 are made of a material having a positive temperature coefficient of resistance. Furthermore, in the gas sensor 5 according to the fifth embodiment, as shown in FIG. 14, the counter electrodes 43 and 44 and the counter electrodes 45 and 46 have interdigitated shapes, whereas the counter electrodes 41 and 42 and the counter electrodes 47 and 48 have linearly opposed shapes. As a result, the opposing length of the counter electrodes 43 and 44 and the counter electrodes 45 and 46 is longer than the opposing length of the counter electrodes 41 and 42 and the counter electrodes 47 and 48. Furthermore, the inter-electrode distance (opposing width) between counter electrodes 43 and 44 and counter electrodes 45 and 46 is shorter than the inter-electrode distance (opposing width) between counter electrodes 41 and 42 and counter electrodes 47 and 48. As a result, when the thermistor films 31 to 34 have substantially the same resistivity and are heated to substantially the same temperature, the resistance value of the thermistor film 31 located between counter electrodes 41 and 42 and the resistance value of the thermistor film 34 located between counter electrodes 47 and 48 are higher than the resistance value of the thermistor film 32 located between counter electrodes 43 and 44 and the resistance value of the thermistor film 33 located between counter electrodes 45 and 46. Since the other basic configuration is the same as that of the gas sensor 1 according to the first embodiment, the same reference numerals are used for the same elements, and redundant description will be omitted.

[0051] The operation of the gas sensor 5 according to the fifth embodiment is the same as that of the gas sensor 1 according to the first embodiment, except that the switches SW1 and SW2 are not controlled. In the gas sensor 5 according to the fifth embodiment, since the switches SW1 and SW2 are not present, the connection point N2 is connected to the wiring to which the power supply voltage Vcc2 is supplied via the detection thermistor Rd4 during the period T1, and the connection point N1 is connected to the wiring to which the power supply voltage Vcc1 is supplied via the detection thermistor Rd1 during the period T2. However, while the thermistor Rd4 is heated to approximately 300°C during period T1 and the thermistor Rd1 is heated to approximately 300°C during period T2, the resistance values ​​of the detection thermistors Rd1 and Rd4 are designed to be in a predetermined resistance range when heated to approximately 150°C. Therefore, the resistance values ​​of the thermistors Rd1 and Rd4 heated to approximately 300°C are sufficiently high because they have positive temperature coefficients of resistance. As a result, the influence of thermistor Rd4 during period T1 and the influence of thermistor Rd1 during period T2 are at a negligible level.

[0052] As exemplified by the gas sensor 5 according to the fifth embodiment, the thermistor films 31 to 34 may be made of a material having a positive temperature coefficient of resistance.

[0053] The above describes embodiments of the technology according to the present disclosure, but the technology according to the present disclosure is not limited to the above embodiments, and various modifications are possible within the scope of the gist of the technology, and it goes without saying that these modifications are also included within the scope of the technology according to the present disclosure.

[0054] For example, in the above embodiment, a thermistor is used as the temperature-sensing element, but the present invention is not limited to this.

[0055] The technology according to the present disclosure includes, but is not limited to, the following configuration examples.

[0056] A gas sensor according to one aspect of the present disclosure comprises a first series circuit including first and second thermosensitive elements connected in series, a second series circuit including third and fourth thermosensitive elements connected in series, a first power supply circuit that applies 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 that appears at a first connection point where the first thermosensitive element and the second thermosensitive element are connected during a first period in which the first and third thermosensitive elements are heated to a first temperature range and the second and fourth thermosensitive elements are heated to a second temperature range, and that detects a second detection voltage that appears at a second connection point where the third thermosensitive element and the fourth thermosensitive element are connected during a second period in which the first and third thermosensitive elements are heated to a second temperature range and the second and fourth thermosensitive elements are heated to the first temperature range, and the signal processing circuit calculates the concentration of a target gas based on the first and second detection voltages. This makes it possible to match the thermal history of the first thermosensing element with the second thermosensing element, and the thermal history of the third thermosensing element with the fourth thermosensing element, and also makes it possible to measure the concentration of the gas to be detected in both the first period and the second period.

[0057] In the gas sensor described above, the resistance values ​​of the first and fourth temperature-sensitive elements may be in a first resistance range when heated to a first temperature range, and the resistance values ​​of the second and third temperature-sensitive elements may be in a second resistance range when heated to a second temperature range. This allows for detection of the concentration of a non-flammable gas, such as CO2 gas, contained in the atmosphere. In this case, the first resistance range and the second resistance range may overlap. This allows for a wide dynamic range to be obtained.

[0058] The gas sensor may further include a first heater for commonly heating the first and third thermosensitive elements and a second heater for commonly heating the second and fourth thermosensitive elements, thereby enabling the first and third thermosensitive elements to be heated to the same temperature, and the second and fourth thermosensitive elements to be heated to the same temperature.

[0059] In the gas sensor described above, the first thermosensitive element may include a first portion of a first thermistor film heated by a first heater and a pair of first counter electrodes facing each other across the first portion of the first thermistor film, the second thermosensitive element may include a first portion of a second thermistor film heated by a second heater and a pair of second counter electrodes facing each other across the first portion of the second thermistor film, the third thermosensitive element may include a first portion of a third thermistor film heated by a first heater and a pair of third counter electrodes facing each other across the first portion of the third thermistor film, and the fourth thermosensitive element may include a first portion of a fourth thermistor film heated by a second heater and a pair of fourth counter electrodes facing each other across the first portion of the fourth thermistor film, thereby improving insulation between the first counter electrode and the third counter electrode and between the second counter electrode and the fourth counter electrode.

[0060] In the above gas sensor, the first thermosensitive element may comprise a first portion of a first thermistor film heated by a first heater and a pair of first opposing electrodes facing each other across the first portion of the first thermistor film, the second thermosensitive element may comprise a first portion of a second thermistor film heated by a second heater and a pair of second opposing electrodes facing each other across the first portion of the second thermistor film, the third thermosensitive element may comprise 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 thermosensitive element may comprise 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, thereby enabling the first to fourth thermosensitive elements to be miniaturized.

[0061] In the above gas sensor, the first to fourth thermosensitive elements may have negative temperature coefficients of resistance, the temperature may be higher in the second temperature range than in the first temperature range, the opposing length of the first opposing electrode may be longer than the opposing length of the second opposing electrode, and the opposing length of the fourth opposing electrode may be longer than the opposing length of the third opposing electrode. This allows the resistance value of the first thermosensitive element to be lower than the resistance value of the second thermosensitive element, and the resistance value of the fourth thermosensitive element to be lower than the resistance value of the third thermosensitive element.

[0062] In the above gas sensor, the first to fourth thermosensitive elements may have negative temperature coefficients of resistance, the temperature in the second temperature range may be higher than the first temperature range, the inter-electrode distance of the first opposing electrodes may be shorter than the inter-electrode distance of the second opposing electrodes, and the inter-electrode distance of the fourth opposing electrodes may be shorter than the inter-electrode distance of the third opposing electrodes. This allows the resistance value of the first thermosensitive element to be lower than the resistance value of the second thermosensitive element, and the resistance value of the fourth thermosensitive element to be lower than the resistance value of the third thermosensitive element.

[0063] In the above gas sensor, the first to fourth thermosensitive elements may have positive temperature coefficients of resistance, the temperature in the second temperature range may be higher than the temperature in the first temperature range, the opposing length of the second opposing electrode may be longer than the opposing length of the first opposing electrode, and the opposing length of the third opposing electrode may be longer than the opposing length of the fourth opposing electrode. This allows the resistance value of the second thermosensitive element to be lower than the resistance value of the first thermosensitive element, and the resistance value of the third thermosensitive element to be lower than the resistance value of the fourth thermosensitive element.

[0064] In the above gas sensor, the first to fourth thermosensitive elements may have positive temperature coefficients of resistance, the temperature in the second temperature range may be higher than the first temperature range, the inter-electrode distance of the second opposing electrodes may be shorter than the inter-electrode distance of the first opposing electrodes, and the inter-electrode distance of the third opposing electrodes may be shorter than the inter-electrode distance of the fourth opposing electrodes. This allows the resistance value of the second thermosensitive element to be lower than the resistance value of the first thermosensitive element, and the resistance value of the third thermosensitive element to be lower than the resistance value of the fourth thermosensitive element.

[0065] In the gas sensor described above, the first to fourth thermosensitive elements may have negative temperature coefficients of resistance, the temperature in the second temperature range may be higher than the first temperature range, the first power supply circuit may include a first switch connected between the first thermosensitive element and a first power supply wiring or ground wiring to which a first power supply voltage is supplied, and the second power supply circuit may include a second switch connected between the fourth thermosensitive element and a second power supply wiring or ground wiring to which a second power supply voltage is supplied. This allows the first thermosensitive element to be disconnected from the first power supply wiring or ground wiring, and the fourth thermosensitive element to be disconnected from the second power supply wiring or ground wiring. In this case, the first connection point and the second connection point are short-circuited, and the signal processing circuit may turn the first switch on and the second switch off during a first period and turn the first switch off and the second switch on during a second period. This allows the fourth thermosensor to be disconnected from the second power supply wiring or ground wiring during the first period, and the first thermosensor to be disconnected from the first power supply wiring or ground wiring during the second period.

[0066] In the gas sensor described above, the first to fourth thermosensitive elements may have a positive temperature coefficient of resistance, the temperature in the second temperature range may be higher than the first temperature range, the first power supply circuit may include a first switch connected between the second thermosensitive element and a first power supply wiring or ground wiring to which a first power supply voltage is supplied, and the second power supply circuit may include a second switch connected between the third thermosensitive element and a second power supply wiring or ground wiring to which a second power supply voltage is supplied. This allows the second thermosensitive element to be disconnected from the first power supply wiring or ground wiring, and the third thermosensitive element to be disconnected from the second power supply wiring or ground wiring. In this case, the first connection point and the second connection point are short-circuited, and the signal processing circuit may turn the first switch on and the second switch off during a first period, and turn the first switch off and the second switch on during a second period. This allows the third thermosensor to be disconnected from the second power supply wiring or ground wiring during the first period, and the second thermosensor to be disconnected from the first power supply wiring or ground wiring during the second period.

[0067] In the above gas sensor, the first power supply circuit may include a first switch connected between the first thermosensitive element and one of a first power supply wiring and a ground wiring to which a first power supply voltage is supplied, and a third switch connected between the second thermosensitive element and the other of the first power supply wiring and the ground wiring, and the second power supply circuit may include a second switch connected between the fourth thermosensitive element and one of a second power supply wiring and a ground wiring to which a second power supply voltage is supplied, and a fourth switch connected between the third thermosensitive element and the other of the second power supply wiring and the ground wiring. This makes it possible to disconnect the first thermosensitive element from one of the first power supply wiring and the ground wiring, disconnect the second thermosensitive element from the other of the first power supply wiring and the ground wiring, disconnect the third thermosensitive element from the other of the second power supply wiring and the ground wiring, and disconnect the fourth thermosensitive element from one of the second power supply wiring and the ground wiring. In this case, the first connection point and the second connection point are short-circuited, and the signal processing circuit may turn on the first and third switches and turn off the second and fourth switches during the first period, and turn off the first and third switches and turn on the second and fourth switches during the second period, so that 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.

[0068] In the above 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, thereby making it possible to cancel the offset even if there is an offset between the first detection voltage and the second detection voltage. [Explanation of symbols]

[0069] 1~5 Gas sensors 10 Signal processing circuit 11,11a,11b amplifier 12 AD converter 13 DA converter 14 Control circuit 20 Substrate 20A, 20B notch 21 insulating film 31~36 Thermistor film 41~48 Counter electrode 51~59 Terminal electrode MH1,MH2 heater resistors N1,N2 connection points Rd1~Rd4 Thermistor S1, S2 sensor section SW1 to SW5 switches Vcc1, Vcc2 power supply voltage Vgas1, Vgas1a, Vgas1b, Vgas2, Vgas2a, Vgas2b detection voltage Vmh1, Vmh2 heater voltage Vref Reference voltage

Claims

1. a first series circuit including first and second temperature sensing elements connected in series; a second series circuit including third and fourth temperature sensitive elements connected in series; a first power supply circuit that applies a voltage to the first series circuit; a second power supply circuit that applies a voltage to the second series circuit; a signal processing circuit that detects a first detection voltage appearing at a first connection point where the first and third temperature sensing elements are connected 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 that detects a second detection voltage appearing at a second connection point where the third and fourth temperature sensing elements are connected 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; Equipped with the signal processing circuit calculates the concentration of the target gas based on the first and second detection voltages. Gas sensor.

2. the resistance values ​​of the first and fourth thermosensitive elements are in a first resistance range when heated to the first temperature range; The resistance values ​​of the second and third temperature sensitive elements become in a second resistance range when heated to the second temperature range.

2. The gas sensor according to claim 1.

3. the first resistance area and the second resistance area have an overlap; 3. The gas sensor according to claim 2.

4. a first heater for commonly heating the first and third thermosensitive elements; a second heater that commonly heats the second and fourth thermosensitive elements; Further provided with 2. The gas sensor according to claim 1.

5. the first thermosensitive element includes a first portion of a first thermistor film heated by the first heater and a pair of first counter electrodes opposed to each other across the first portion of the first thermistor film; the second thermosensitive element includes a first portion of a second thermistor film heated by the second heater and a pair of second counter electrodes opposed to each other across the first portion of the second thermistor film; the third thermosensitive element includes a first portion of a third thermistor film heated by the first heater and a pair of third opposing electrodes opposed to each other across the first portion of the third thermistor film; the fourth thermosensitive element includes a first portion of a fourth thermistor film heated by the second heater and a pair of fourth opposing electrodes opposed to each other across the first portion of the fourth thermistor film; 5. The gas sensor according to claim 4.

6. the first thermosensitive element includes a first portion of a first thermistor film heated by the first heater and a pair of first counter electrodes opposed to each other across the first portion of the first thermistor film; the second thermosensitive element includes a first portion of a second thermistor film heated by the second heater and a pair of second counter electrodes opposed to each other across the first portion of the second thermistor film; the third thermosensitive element includes a second portion of the first thermistor film and a pair of third opposing electrodes opposed to each other across the second portion of the first thermistor film, the fourth thermosensitive element includes a second portion of the second thermistor film and a pair of fourth opposing electrodes opposed to each other across the second portion of the second thermistor film; 5. The gas sensor according to claim 4.

7. the first to fourth temperature sensitive elements have a negative temperature coefficient of resistance, the temperature in the second temperature range is higher than the temperature in the first temperature range, a length over which the first opposing electrodes are opposed is longer than a length over which the second opposing electrodes are opposed; a length over which the fourth opposing electrode faces is longer than a length over which the third opposing electrode faces; 7. The gas sensor according to claim 5.

8. the first to fourth temperature sensitive elements have a negative temperature coefficient of resistance, the temperature in the second temperature range is higher than the temperature in the first temperature range, an inter-electrode distance of the first opposing electrodes is shorter than an 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; 7. The gas sensor according to claim 5.

9. the first to fourth temperature sensitive elements have a positive temperature coefficient of resistance, the temperature in the second temperature range is higher than the temperature in the first temperature range, a length over which the second opposing electrode faces is longer than a length over which the first opposing electrode faces; a length over which the third opposing electrode faces is longer than a length over which the fourth opposing electrode faces; 7. The gas sensor according to claim 5.

10. the first to fourth temperature sensitive elements have a positive temperature coefficient of resistance, the temperature in the second temperature range is higher than the temperature in the first temperature range, an inter-electrode distance of the second opposing electrodes is shorter than an 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; 7. The gas sensor according to claim 5.

11. the first to fourth temperature sensitive elements have a negative temperature coefficient of resistance, the temperature in the second temperature range is higher than the temperature in the first temperature range, the first power supply circuit includes a first switch connected between a first power supply wiring or a ground wiring to which a first power supply voltage is supplied and the first temperature sensing element; the second power supply circuit includes a second switch connected between a second power supply wiring or a ground wiring to which a second power supply voltage is supplied and the fourth temperature sensing element; 2. The gas sensor according to claim 1.

12. the first to fourth temperature sensitive elements have a positive temperature coefficient of resistance, the temperature in the second temperature range is higher than the temperature in the first temperature range, the first power supply circuit includes a first switch connected between a first power supply wiring or a ground wiring to which a first power supply voltage is supplied and the second temperature sensing element; the second power supply circuit includes a second switch connected between a second power supply wiring or a ground wiring to which a second power supply voltage is supplied and the third temperature sensing element; 2. The gas sensor according to claim 1.

13. 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; 13. The gas sensor according to claim 11 or 12.

14. the first power supply circuit includes a first switch connected between one of a first power supply wiring and a ground wiring to which a first power supply voltage is supplied and the first thermosensitive element, and a third switch connected between the other of the first power supply wiring and the ground wiring and the second thermosensitive element; the second power supply circuit includes a second switch connected between one of a second power supply wiring to which a second power supply voltage is supplied and the ground wiring and the fourth temperature sensing element, and a fourth switch connected between the other of the second power supply wiring and the ground wiring and the third temperature sensing element; 2. The gas sensor according to claim 1.

15. 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; 15. The gas sensor according to claim 14.

16. the signal processing circuit calculates a moving average value 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; 2. The gas sensor according to claim 1.

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