Gas sensor

Abstract

The present disclosure provides a gas sensor having further improved detection sensitivity. A gas sensor (100) is provided with: heaters (MH1, MH2); a thermistor (Rd1), the temperature of which changes according to the change of the temperature of the heater (MH1); a thermistor (Rd2), the temperature of which changes in accordance with a change in the temperature of the heater (MH2); and a control circuit (46). When gas concentration measurement is performed, the heater (MH1) is heated to a first temperature region, and the heater (MH2) is heated to a second temperature region that is higher than the temperature of the first temperature region. When the gas concentration is measured, the direction of the temperature change of the thermistor (Rd1) is opposite to the direction of the temperature change of the thermistor (Rd2) with respect to the increase in the concentration of the gas to be measured. The control circuit (46) calculates the concentration of the gas to be measured on the basis of a gas detection signal (Vgas) occurring at a connection point (N1) between the thermistor (Rd1) and the thermistor (Rd2).

Description

Technical Field

[0001] This disclosure relates to gas sensors, and more particularly to gas sensors with high detection sensitivity. Background Technology

[0002] Patent document 1 discloses a gas sensor that can reduce the influence of gases outside the target gas by measuring the concentration of the target gas under two conditions: a temperature region with high detection sensitivity and a temperature region with low detection sensitivity.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent No. 6879060 Summary of the Invention

[0006] The technical problem the invention aims to solve

[0007] However, in the gas sensor described in Patent Document 1, since the sensitivity direction of the thermistor in the temperature region with high detection sensitivity is the same as that of the thermistor in the temperature region with low detection sensitivity, the detection sensitivity will decrease by an amount corresponding to the change in the characteristics of the thermistor obtained in the temperature region with low detection sensitivity.

[0008] This disclosure describes a gas sensor with further improved detection sensitivity.

[0009] Means for solving technical problems

[0010] One aspect of this disclosure includes a gas sensor comprising: a first heater; a second heater; a first temperature-sensing element whose temperature varies according to a change in the temperature of the first heater; a second temperature-sensing element whose temperature varies according to a change in the temperature of the second heater; and a control circuit, wherein the first temperature-sensing element and the second temperature-sensing element are connected in series, and during gas concentration measurement, the first heater is heated to a first temperature region, and the second heater is heated to a second temperature region with a temperature higher than the first temperature region, wherein during gas concentration measurement, the direction of temperature change of the first temperature-sensing element is opposite to the direction of temperature change of the second temperature-sensing element relative to an increase in the concentration of the gas to be measured, and the control circuit calculates the concentration of the gas to be measured based on a gas detection signal appearing at the connection point of the first temperature-sensing element and the second temperature-sensing element.

[0011] Another aspect of this disclosure includes a gas sensor comprising: a first heater; a second heater; a first temperature-sensing element whose temperature varies according to a change in the temperature of the first heater; a second temperature-sensing element whose temperature varies according to a change in the temperature of the second heater; and a control circuit, wherein, during gas concentration measurement, the first heater is heated to a first temperature region, and the second heater is heated to a second temperature region higher than the first temperature region; during gas concentration measurement, the direction of temperature change of the first temperature-sensing element is opposite to the direction of temperature change of the second temperature-sensing element relative to an increase in the concentration of the gas to be measured; and the control circuit calculates the concentration of the gas to be measured based on the difference between a first output voltage caused by the first temperature-sensing element and a second output voltage caused by the second temperature-sensing element.

[0012] Invention Effects

[0013] According to this disclosure, a gas sensor with further improved detection sensitivity can be provided. Attached Figure Description

[0014] Figure 1 This is a circuit diagram showing the structure of a gas sensor 100 according to a first embodiment of the technology disclosed herein.

[0015] Figure 2 It is a graph used to illustrate the change in the thermal conductivity of CO2 gas relative to air caused by temperature.

[0016] Figure 3 This is a graph illustrating an example of the change in resistance values ​​of thermistors Rd1 and Rd2 in response to changes in the concentration of CO2 gas in the measured atmosphere.

[0017] Figure 4 This is a timing diagram used to illustrate the operation of the gas sensor 100.

[0018] Figure 5 This is a circuit diagram showing the structure of a gas sensor 200 according to a second embodiment of the technology disclosed herein.

[0019] Figure 6 This is a circuit diagram showing the structure of a gas sensor 300 according to a third embodiment of the technology disclosed herein.

[0020] Symbol Explanation

[0021] 10, 20 Sensors

[0022] 30 Temperature Sensor

[0023] 40 Signal Processing Circuit

[0024] 41, 42 Differential Amplifiers

[0025] 43 Buffer

[0026] 44 AD converter

[0027] 45 DA Converter

[0028] 46 Control Circuit

[0029] 47-49 Differential Amplifiers

[0030] 50 Sensors

[0031] 100, 200, 300 gas sensors

[0032] MH1 and MH2 heaters

[0033] N1~N5 connection points

[0034] Fixed resistors R1 to R5

[0035] Thermistors Rd1~Rd3

[0036] TP1 and TP2 thermopile elements

[0037] Vamp1 and Vamp2 amplify the signal

[0038] Vgas, Vgas1, Vgas2 gas detection signals

[0039] Vmh1, Vmh2 heater voltage

[0040] Vout output signal

[0041] Vref, Vref1, Vref2 reference potentials

[0042] Vtemp temperature detection signal

[0043] Vtp1 and Vtp2 output signals Detailed Implementation

[0044] Hereinafter, embodiments of the technology involved in this disclosure will be described in detail with reference to the accompanying drawings.

[0045] <First Implementation Method>

[0046] Figure 1 This is a circuit diagram illustrating the structure of a gas sensor 100 according to a first embodiment of the technology disclosed herein.

[0047] like Figure 1As shown, the gas sensor 100 of the first embodiment includes: a sensor unit 10 that generates a gas detection signal Vgas corresponding to the concentration of the gas to be measured; a temperature sensor 30 that generates a temperature detection signal Vtemp corresponding to the ambient temperature; and a signal processing circuit 40. Although not particularly limited, the gas sensor 100 of the first embodiment is a thermally conductive gas sensor for detecting the concentration of CO2 gas in a measurement atmosphere.

[0048] The sensor unit 10 includes thermistors Rd1 and Rd2 and heaters MH1 and MH2 connected in series between the power supply Vcc and ground GND. The temperature of thermistor Rd1 changes according to the temperature of heater MH1, and the temperature of thermistor Rd2 changes according to the temperature of heater MH2. The gas detection signal Vgas output from the sensor unit 10 appears at the connection point N1 of thermistors Rd1 and Rd2. Thermistors Rd1 and Rd2 are resistive elements whose resistance changes with temperature. Examples of materials for thermistors Rd1 and Rd2 include vanadium oxide, amorphous silicon, polycrystalline silicon, oxides with a spinel-type crystal structure containing manganese, titanium oxide, or yttrium-barium-copper oxide.

[0049] Figure 2 It is a graph used to illustrate the change in the thermal conductivity of CO2 gas relative to air caused by temperature.

[0050] like Figure 2As shown, at room temperature (25°C), the thermal conductivity of CO2 gas is lower than that of air (thermal conductivity ratio < 1). However, as the temperature increases, the thermal conductivity ratio increases, and if a specified temperature is exceeded, the thermal conductivity of CO2 gas becomes higher than that of air (thermal conductivity ratio > 1). The threshold temperature at which the relationship between the thermal conductivity of CO2 gas and that of air reverses, i.e., the temperature at which the thermal conductivity ratio becomes 1, is approximately 400°C. Furthermore, during the gas concentration measurement operation, heater MH1 is heated to a temperature below the threshold temperature, for example, around 150°C (an example of the first temperature region), and heater MH2 is heated to a temperature above the threshold temperature, for example, around 430°C (an example of the second temperature region). The first temperature region, which is the heating temperature of heater MH1, is, for example, a specified temperature region encompassing a range of 100°C or higher and 300°C or lower. The second temperature region, which is the heating temperature of heater MH2, is, for example, a specified temperature region encompassing a range of 400°C or higher and 450°C or lower. The term "temperature region" in this specification, for example, has a temperature range of less than 1°C. For example, the temperature range near 150°C can be a region above 149.5°C and below 150.5°C, and the temperature range near 430°C can be a region above 429.5°C and below 430.5°C. When the thermistor Rd1 is located near the heater MH1, when the heater MH1 is heated to 150°C, the thermistor Rd1 is also heated to approximately 150°C. When the thermistor Rd2 is located near the heater MH2, when the heater MH2 is heated to 430°C, the thermistor Rd2 is also heated to approximately 430°C. Thermistor Rd1 is designed to have a predetermined resistance value when heated to 150°C, and the thermistor Rd2 is designed to have a predetermined resistance value when heated to 430°C. This design can be achieved by adjusting the width and distance between the pair of electrodes of the thermistor Rd1, and the width and distance between the pair of electrodes of the thermistor Rd2.

[0051] If CO2 gas is present in the measuring atmosphere while heaters MH1 and MH2 are heated, the heat dissipation characteristics of heaters MH1 and MH2 will change accordingly with their concentration. This change manifests as a change in the temperature of thermistors Rd1 and Rd2, i.e., a change in the resistance values ​​of thermistors Rd1 and Rd2.

[0052] If used Figure 2As explained, in the temperature range below approximately 400°C, which is the threshold temperature, the thermal conductivity of CO2 gas is lower than that of air (thermal conductivity ratio < 1). Therefore, if CO2 gas is present in the measuring atmosphere while the heater MH1 is being heated by applying a certain electrical power, the higher the concentration of CO2 gas, the higher the temperature of the heater MH1. Consequently, the temperature of the thermistor Rd1 also increases. Therefore, if the heater MH1 is heated to 150°C when the CO2 gas concentration in the measuring atmosphere is the same as the concentration of CO2 gas in the atmosphere under normal atmospheric conditions (e.g., 400 ppm), and the concentration of CO2 gas in the measuring atmosphere exceeds the concentration of CO2 gas in the atmosphere under normal atmospheric conditions, the temperature of the heater MH1 becomes higher than 150°C accordingly, and the temperature of the thermistor Rd1 also becomes higher than when the CO2 gas concentration in the measuring atmosphere is the same as the concentration of CO2 gas in the atmosphere under normal atmospheric conditions (e.g., 400 ppm). As a result, for example, when the thermistor Rd1 has a negative temperature coefficient of resistance (when the thermistor Rd1 is an NTC thermistor), the higher the concentration of CO2 gas in the measured atmosphere, the lower the resistance value of the thermistor Rd1.

[0053] Conversely, in the temperature range above approximately 400°C (the threshold temperature), the thermal conductivity of CO2 gas is higher than that of air (thermal conductivity ratio > 1). Therefore, if CO2 gas is present in the measuring atmosphere while the heater MH2 is being heated by applying a certain electrical power, the higher the concentration of CO2 gas, the lower the temperature of the heater MH2. Consequently, the temperature of the thermistor Rd2 also decreases. Therefore, if the heater MH2 is heated to 430°C when the CO2 gas concentration in the measuring atmosphere is the same as the normal atmospheric CO2 gas concentration (e.g., 400 ppm), then when the CO2 gas concentration in the measuring atmosphere exceeds the normal atmospheric CO2 gas concentration, the temperature of the heater MH2 becomes lower than 430°C, and the temperature of the thermistor Rd2 also becomes lower than when the CO2 gas concentration in the measuring atmosphere is the same as the normal atmospheric CO2 gas concentration (e.g., 400 ppm). As a result, for example, when the thermistor Rd2 has a negative temperature coefficient of resistance (when the thermistor Rd2 is an NTC thermistor), the higher the concentration of CO2 gas in the measured atmosphere, the higher the resistance value of the thermistor Rd2.

[0054] Furthermore, thermistors Rd1 and Rd2 are connected in series between the power supply Vcc and ground GND. Therefore, the higher the CO2 gas concentration in the measured atmosphere, the higher the level of the gas detection signal Vgas appearing at the connection point N1.

[0055] Figure 3 This is a graph illustrating an example of the change in resistance values ​​of thermistors Rd1 and Rd2 in response to changes in the concentration of CO2 gas in the measured atmosphere.

[0056] exist Figure 3 In the example shown, the CO2 gas concentration in the measured atmosphere is 400 ppm before time t1, 2500 ppm from time t1 to time t2, 5000 ppm from time t2 to time t3, and 400 ppm after time t3. Furthermore, if the resistance values ​​of thermistors Rd1 (the resistance between the two electrodes of thermistor Rd1) and Rd2 (the resistance between the two electrodes of thermistor Rd2) are both r0 before time t1, then during the period from time t1 to time t2, the resistance value of thermistor Rd1 decreases to r1 (< r0), and the resistance value of thermistor Rd2 increases to r2 (> r0). As a result, the level of the gas detection signal Vgas appearing at connection point N1 rises to a level corresponding to the ratio of resistance values ​​r1 and r2. Specifically, Vgas = Vcc / (1 + r1 / r2). At this time, the difference between resistance values ​​r1 and r2 is Δr12. Furthermore, during the period from time t2 to time t3, the resistance value of thermistor Rd1 further decreases to r3 (< r1), while the resistance value of thermistor Rd2 further increases to r4 (> r2). As a result, the level of the gas detection signal Vgas appearing at connection point N1 rises to a level corresponding to the ratio of resistance values ​​r3 and r4. Specifically, Vgas = Vcc / (1 + r3 / r4). At this time, the difference between resistance values ​​r3 and r4 is Δr34, which is greater than the difference Δr12. Additionally, the ratio of resistance value r3 to resistance value r4, r3 / r4, is less than the ratio of resistance value r1 to resistance value r2, r1 / r2.

[0057] Thus, during gas concentration measurement, the temperature change of thermistor Rd1 is in the opposite direction to that of thermistor Rd2 relative to the increase in CO2 gas concentration. Consequently, the resistance of thermistor Rd1 changes in the opposite direction to that of thermistor Rd2 relative to the increase in CO2 gas concentration. Therefore, compared to the case where the resistance changes in the same direction, the ratio of the resistance of thermistor Rd1 to the resistance of thermistor Rd2 changes more significantly with the increase in CO2 gas concentration. Consequently, the level of the gas detection signal Vgas appearing at connection point N1 changes even more significantly with the increase in CO2 gas concentration.

[0058] However, when the CO2 gas concentration in the measuring atmosphere is the same as that in normal atmospheric CO2 (e.g., 400 ppm), the resistance values ​​of thermistors Rd1 and Rd2 do not need to be the same (=r0), and they can be different. If the CO2 gas concentration in the measuring atmosphere is the same as that in normal atmospheric CO2 (e.g., 400 ppm), then the level of the gas detection signal Vgas becomes Vcc / 2.

[0059] On the other hand, even if the measuring atmosphere contains other gases whose heat dissipation characteristics are not significantly different when heating heater MH1 to around 150°C compared to those when heating heater MH2 to around 430°C, the concentration of these gases has almost no effect on the level of the gas detection signal Vgas. Therefore, the sensor unit 10 can selectively detect the concentration of CO2 gas.

[0060] Temperature sensor 30 includes a thermistor Rd3 and a fixed resistor R3 connected in series between the power supply Vcc and ground GND. The temperature detection signal Vtemp output from temperature sensor 30 appears at the connection point N2 of the thermistor Rd3 and the fixed resistor R3. Temperature sensor 30 detects the ambient temperature. Ambient temperature refers to the temperature of the measured atmosphere. Temperature sensor 30 can also be designed to be unaffected by heating from heaters MH1 and MH2, or to be less susceptible to heating from heaters MH1 and MH2.

[0061] The signal processing circuit 40 includes a differential amplifier 41, a buffer 43, an AD converter (ADC) 44, a DA converter (DAC) 45, and a control circuit 46.

[0062] Differential amplifier 41 generates an amplified signal Vamp1 by comparing the gas detection signal Vgas with the reference potential Vref, which is the level difference between the gas detection signal Vgas and the reference potential Vref (=Vgas-Vref). Buffer 43 generates an amplified signal Vamp2 by buffering the temperature detection signal Vtemp. Amplified signals Vamp1 and Vamp2 are input to AD converter 44. AD converter 44 generates digital values ​​by performing AD conversion on the amplified signals Vamp1 and Vamp2, and provides the digital values ​​to control circuit 46.

[0063] The control circuit 46 calculates the concentration of CO2 gas, the target gas, based on the amplified signal Vamp1 after AD conversion, and generates an output signal Vout indicating the CO2 gas concentration. The calculation of the CO2 gas concentration can also use a formula set within the control circuit 46. Furthermore, the control circuit 46 supplies digital values ​​of various control parameters to the DA converter 45. The DA converter 45 generates heater voltages Vmh1 and Vmh2 and a reference potential Vref by performing analog-to-analog conversion on the digital values ​​of the various control parameters. The heater voltages Vmh1 and Vmh2 are applied to heaters MH1 and MH2, respectively, thereby heating heaters MH1 and MH2. Additionally, the reference potential Vref is supplied to the differential amplifier 41.

[0064] Figure 4 This is a timing diagram used to illustrate the operation of the gas sensor 100.

[0065] like Figure 4 As shown, in this embodiment, the gas sensor 100 heats heaters MH1 and MH2 simultaneously by applying heater voltages Vmh1 and Vmh2 when measuring gas concentration. The heating temperature of heater MH1 caused by the application of heater voltage Vmh1 is below a threshold temperature, for example, 150°C. The heating temperature of heater MH2 caused by the application of heater voltage Vmh2 is above the threshold temperature, for example, 430°C. The levels of heater voltages Vmh1 and Vmh2 are adjusted by referring to the amplified signal Vamp2 obtained by amplifying the temperature signal Vtemp, so that the temperatures of heaters MH1 and MH2 are respectively set to predetermined values ​​independent of the ambient temperature. For example, when the CO2 gas concentration in the measurement environment is the normal concentration of CO2 gas in the atmosphere (for example, 400 ppm), the level of heater voltage Vmh1 is set such that the heating temperature of heater MH1 is, for example, 150°C, and the level of heater voltage Vmh2 is set such that the heating temperature of heater MH2 is, for example, 430°C. There is no particular limitation on the heating temperature of heater MH1 as long as it is below the threshold temperature. Figure 2 As explained, in the temperature range below the threshold temperature, the thermal conductivity ratio approaches 1 as the temperature increases. Therefore, by setting the heating temperature of heater MH1 below 300°C, high detection sensitivity can be obtained.

[0066] Then, while heaters MH1 and MH2 are simultaneously heated, the gas detection signal Vgas is input into the signal processing circuit 40, Vout is calculated based on its level, and then output to the outside. By periodically performing such gas concentration measurements, changes in the concentration of CO2 gas in the measurement atmosphere can be periodically detected.

[0067] As explained above, when the gas sensor 100 of this embodiment measures gas concentration, the direction of temperature change of the thermistor Rd1 is opposite to the direction of temperature change of the thermistor Rd2 relative to the increase of CO2 gas concentration. As a result, when measuring gas concentration, the direction of resistance change of the thermistor Rd1 is opposite to the direction of resistance change of the thermistor Rd2 relative to the increase of CO2 gas concentration. Therefore, the change in the level of the gas detection signal Vgas corresponding to the change in CO2 gas concentration becomes larger. This further improves the detection sensitivity of CO2 gas concentration.

[0068] <Second Implementation Method>

[0069] Figure 5 This is a circuit diagram illustrating the structure of a gas sensor 200 according to a second embodiment of the technology disclosed herein.

[0070] like Figure 5 As shown, the gas sensor 200 of the second embodiment differs from the gas sensor 100 of the first embodiment in that the sensor unit 10 is replaced by the sensor unit 20, and the differential amplifier 41 included in the signal processing circuit 40 is replaced by the differential amplifier 42. Other basic structures are the same as those of the gas sensor 100 of the first embodiment; therefore, the same reference numerals are used to label the same elements, and repeated descriptions are omitted.

[0071] The sensor unit 20 includes a thermistor Rd1 and a fixed resistor R1 connected in series between the power supply Vcc and ground GND, a thermistor Rd2 and a fixed resistor R2 connected in series between the power supply Vcc and ground GND, and heaters MH1 and MH2. The temperature of thermistor Rd1 changes according to the temperature of heater MH1, and the temperature of thermistor Rd2 changes according to the temperature of heater MH2. Gas detection signal Vgas1 appears at the connection point N3 of thermistor Rd1 and fixed resistor R1, and gas detection signal Vgas2 appears at the connection point N4 of thermistor Rd2 and fixed resistor R2. The resistance values ​​of fixed resistors R1 and R2 can also be close to the resistance values ​​of thermistors Rd1 and Rd2 when the CO2 gas concentration in the measured atmosphere is the normal atmospheric CO2 gas concentration (e.g., 400 ppm).

[0072] The differential amplifier 42 included in the signal processing circuit 40 compares the gas detection signal Vgas1 with the gas detection signal Vgas2 and generates an amplified signal Vamp1 that amplifies the horizontal difference (=Vgas1-Vgas2) between the gas detection signals Vgas1 and Vgas2.

[0073] If using Figure 3 To illustrate with the example shown, if the resistance values ​​of thermistors Rd1 and Rd2 are both r0 before time t1, then between time t1 and time t2, the resistance value of thermistor Rd1 decreases to r1 (< r0), and the resistance value of thermistor Rd2 increases to r2 (> r0). As a result, the level of the gas detection signal Vgas1 at connection point N3 increases, and the level of the gas detection signal Vgas2 at connection point N4 decreases. The difference between the levels of gas detection signals Vgas1 and Vgas2 corresponds to the difference Δr12 between resistance values ​​r1 and r2. Furthermore, between time t2 and time t3, the resistance value of thermistor Rd1 further decreases to r3 (< r1), and the resistance value of thermistor Rd2 further increases to r4 (> r2). As a result, the level of the gas detection signal Vgas1 at connection point N3 further increases, and the level of the gas detection signal Vgas2 at connection point N4 further decreases. The level difference between gas detection signals Vgas1 and Vgas2 corresponds to the difference Δr34 between resistance values ​​r3 and r4.

[0074] Thus, during gas concentration measurement, the temperature change direction of thermistor Rd1 is opposite to that of thermistor Rd2 relative to the increase in CO2 gas concentration. Consequently, the resistance value of thermistor Rd1 changes in the opposite direction to that of thermistor Rd2 relative to the increase in CO2 gas concentration. Therefore, compared to the case where the resistance values ​​change in the same direction, the differences Δr12 and Δr34 are increased. Consequently, the change in the level difference between gas detection signals Vgas1 and Vgas2 (=Vgas1-Vgas2) relative to the increase in CO2 gas concentration is further amplified.

[0075] As illustrated in the gas sensor 200 of the second embodiment, it is not necessary to connect the thermistors Rd1 and Rd2 in series between the power supply Vcc and ground GND. They can also be connected in parallel between the power supply Vcc and ground GND. The concentration of the gas to be measured is calculated based on the difference between the output voltage (gas detection signal Vgas1) caused by the thermistor Rd1 and the output voltage (gas detection signal Vgas2) caused by the thermistor Rd2.

[0076] As explained above, in this embodiment, when the gas sensor 200 measures gas concentration, the direction of temperature change of the thermistor Rd1 is opposite to the direction of temperature change of the thermistor Rd2 relative to the increase in CO2 gas concentration. As a result, since the direction of resistance change of the thermistor Rd1 is opposite to the direction of resistance change of the thermistor Rd2 relative to the increase in CO2 gas concentration during gas concentration measurement, the level difference (=Vgas1-Vgas2) between the gas detection signals Vgas1 and Vgas2 increases accordingly with the change in CO2 gas concentration. Therefore, the detection sensitivity of CO2 gas concentration can be further improved.

[0077] <Third Implementation Method>

[0078] Figure 6 This is a circuit diagram illustrating the structure of a gas sensor 300 according to a third embodiment of the technology disclosed herein.

[0079] like Figure 6 As shown, the gas sensor 300 of the third embodiment differs from the gas sensor 200 of the second embodiment in that the sensor section 20 is replaced by the sensor section 50, and differential amplifiers 47 to 49 are provided in the signal processing circuit 40. Other basic structures are the same as those of the gas sensor 200 of the second embodiment; therefore, the same reference numerals are used to label the same elements, and repeated descriptions are omitted.

[0080] The sensor unit 50 includes thermopile elements TP1 and TP2 and heaters MH1 and MH2. The temperature of the hot junction of thermopile element TP1 changes according to the temperature of heater MH1, and the temperature of the hot junction of thermopile element TP2 changes according to the temperature of heater MH2. The potential difference between the two ends of thermopile elements TP1 and TP2 changes according to the temperature. The potential difference between the two ends of thermopile element TP1 is used as the output signal Vtp1, and the potential difference between the two ends of thermopile element TP2 is used as the output signal Vtp2. The reference potential Vref1 is generated by fixed resistors R4 and R5. Fixed resistors R4 and R5 are connected in series between the power supply Vcc and ground GND, and the reference potential Vref1 appears at their connection point N5. The reference potential Vref1 and the output signals Vtp1 and Vtp2 are supplied to the signal processing circuit 40. The potential supplied to the non-inverting input terminal (+) of differential amplifier 48 is a level obtained by superimposing the output signal Vtp1, which corresponds to the electromotive force of thermopile element TP1 at the corresponding temperature, onto the reference potential Vref1. The potential supplied to the non-inverting input terminal (+) of differential amplifier 49 is a level obtained by superimposing the output signal Vtp2, which corresponds to the electromotive force of thermopile element TP2 at the corresponding temperature, onto the reference potential Vref1.

[0081] The output signal Vtp1 is amplified by the differential amplifier 48 included in the signal processing circuit 40 to generate the gas detection signal Vgas1. The differential amplifier 48 compares the reference potential Vref1 supplied to the inverting input terminal (-) with the level of Vref1+Vtp1 supplied to the non-inverting input terminal (+), and generates the gas detection signal Vgas1 by amplifying the difference (=Vtp1).

[0082] The output signal Vtp2 is amplified by the differential amplifier 49 included in the signal processing circuit 40 to generate the gas detection signal Vgas2. The differential amplifier 49 compares the reference potential Vref1 supplied to the inverting input terminal (-) with the level of Vref1+Vtp2 supplied to the non-inverting input terminal (+), and generates the gas detection signal Vgas2 by amplifying the difference (=Vtp2).

[0083] The differential amplifier 42, like in the second embodiment, compares the gas detection signals Vgas1 and Vgas2 to generate an amplified signal Vamp1, which is obtained by amplifying the level difference (=Vgas1-Vgas2) between the gas detection signals Vgas1 and Vgas2.

[0084] In this embodiment, the temperature detection signal Vtemp output from the temperature sensor 30 is supplied to the differential amplifier 47 included in the signal processing circuit 40. The differential amplifier 47 generates an amplified signal Vamp2 by comparing the temperature detection signal Vtemp with a reference potential Vref2, which is the amplified difference between the temperature detection signal Vtemp and the reference potential Vref2 (=Vtemp-Vref2). Alternatively, as in the gas sensor 100 of the first embodiment and the gas sensor 200 of the second embodiment, the temperature detection signal Vtemp can be buffered using a buffer 43 to generate the amplified signal Vamp2.

[0085] Even with this circuit structure, when the CO2 gas concentration in the measuring atmosphere increases, the temperature of the hot junction of thermopile element TP1 rises during gas concentration measurement, thus increasing the level of the gas detection signal Vgas1, which corresponds to the thermoelectric potential of thermopile element TP1. Conversely, the temperature of the hot junction of thermopile element TP2 decreases during gas concentration measurement, thus decreasing the level of the gas detection signal Vgas2, which corresponds to the thermoelectric potential of thermopile element TP2. Therefore, during gas concentration measurement, the direction of temperature change at the hot junction of thermopile element TP1 is opposite to that of the hot junction of thermopile element TP2 relative to the increase in CO2 gas concentration. Consequently, compared to the case where the temperature changes in the same direction, the difference in level between gas detection signals Vgas1 and Vgas2 (=Vgas1-Vgas2) relative to the increase in CO2 gas concentration becomes larger.

[0086] As illustrated in the gas sensor 300 of the third embodiment, the use of a thermistor as a temperature sensing element is not mandatory; other types of temperature sensing elements, such as thermopile elements, can also be used. Furthermore, as illustrated in the gas sensor 300 of the third embodiment, the concentration of the target gas can be calculated based on the difference between the output voltage (gas detection signal Vgas1) caused by the thermopile element TP1 and the output voltage (gas detection signal Vgas2) caused by the thermopile element TP2.

[0087] As explained above, in this embodiment, when the gas sensor 300 measures gas concentration, the direction of temperature change at the hot junction of thermopile element TP1 is opposite to the direction of temperature change at the hot junction of thermopile element TP2 relative to the increase in CO2 gas concentration. As a result, when measuring gas concentration, the direction of change in the thermoelectric potential of thermopile element TP1 is opposite to the direction of change in the thermoelectric potential of thermopile element TP2 relative to the increase in CO2 gas concentration. Therefore, the difference in level between gas detection signals Vgas1 and Vgas2 (=Vgas1-Vgas2) increases accordingly with the change in CO2 gas concentration. This further improves the detection sensitivity of CO2 gas concentration.

[0088] The above describes the implementation of the technology involved in this disclosure. However, the technology involved in this disclosure is not limited to the above implementation, and various changes can be made without departing from its spirit. These changes are of course also included in the scope of the technology involved in this disclosure.

[0089] The technology involved in this disclosure includes, but is not limited to, the following configuration examples.

[0090] One aspect of this gas sensor includes: a first heater; a second heater; a first temperature-sensing element whose temperature varies according to a change in the temperature of the first heater; a second temperature-sensing element whose temperature varies according to a change in the temperature of the second heater; and a control circuit. The first and second temperature-sensing elements are connected in series. During gas concentration measurement, the first heater is heated to a first temperature region, and the second heater is heated to a second temperature region higher than the first temperature region. During gas concentration measurement, the direction of temperature change of the first temperature-sensing element is opposite to the direction of temperature change of the second temperature-sensing element relative to an increase in the concentration of the target gas. The control circuit calculates the concentration of the target gas based on a gas detection signal appearing at the connection point of the first and second temperature-sensing elements. Therefore, high detection sensitivity for the target gas can be obtained. Furthermore, the gas detection signal can be obtained using a half-bridge circuit including the first and second temperature-sensing elements.

[0091] Another aspect of this disclosure describes a gas sensor comprising: a first heater; a second heater; a first temperature-sensing element whose temperature varies according to a change in the temperature of the first heater; a second temperature-sensing element whose temperature varies according to a change in the temperature of the second heater; and a control circuit. During gas concentration measurement, the first heater is heated to a first temperature region, and the second heater is heated to a second temperature region higher than the first temperature region. During gas concentration measurement, the direction of temperature change of the first temperature-sensing element is opposite to the direction of temperature change of the second temperature-sensing element relative to an increase in the concentration of the target gas. The control circuit calculates the concentration of the target gas based on the difference between a first output voltage caused by the first temperature-sensing element and a second output voltage caused by the second temperature-sensing element. This allows for high detection sensitivity of the target gas. Furthermore, the level of the second output voltage can be adjusted without involving the first temperature-sensing element, and the level of the first output voltage can be adjusted without involving the second temperature-sensing element.

[0092] In the gas sensor described above, the target gas can be CO2 gas, with a first temperature region defined as a temperature range below 300°C and a second temperature region defined as a temperature range above 400°C. This allows for the high-sensitivity detection of the CO2 gas concentration in the measurement atmosphere.

Claims

1. A gas sensor, wherein, have: First heater; Second heater; A first temperature-sensing element whose temperature changes according to the temperature of the first heater; A second temperature-sensing element whose temperature changes according to the temperature of the second heater; and Control circuit, The first temperature sensing element and the second temperature sensing element are connected in series. During gas concentration measurement, the first heater is heated to a first temperature region, and the second heater is heated to a second temperature region that is higher than the first temperature region. During the gas concentration measurement, the direction of temperature change of the first temperature sensing element is opposite to the direction of temperature change of the second temperature sensing element relative to the increase in the concentration of the gas being measured. The control circuit calculates the concentration of the gas to be measured based on the gas detection signal that appears at the connection point between the first temperature sensing element and the second temperature sensing element.

2. A gas sensor, wherein, have: First heater; Second heater; A first temperature-sensing element whose temperature changes according to the temperature of the first heater; A second temperature-sensing element whose temperature changes according to the temperature of the second heater; and Control circuit, During gas concentration measurement, the first heater is heated to a first temperature region, and the second heater is heated to a second temperature region that is higher than the first temperature region. During the gas concentration measurement, the direction of temperature change of the first temperature sensing element is opposite to the direction of temperature change of the second temperature sensing element relative to the increase in the concentration of the gas being measured. The control circuit calculates the concentration of the gas to be measured based on the difference between the first output voltage caused by the first temperature sensing element and the second output voltage caused by the second temperature sensing element.

3. The gas sensor as described in claim 1 or 2, wherein, The gas being measured is CO2. The first temperature range is a specified temperature range below 300°C. The second temperature zone is a specified temperature zone exceeding 400°C.

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