Gas sensor

Abstract

To make a measurement result follow a change in density even when detection object gas largely changes for a short period.SOLUTION: A gas sensor 100 comprises: sensor units 10 and 20 that output detection signals Vgas1 and Vgas2, respectively; and a signal processing circuit 40 that calculates density of the detection object gas on the basis of the detection signals Vgas1 and Vgas2. The signal processing circuit 40 is configured to, when controlling the sensor unit 10, alternately repeat implementation of a gas density measurement action and a dummy heating action, and when controlling the sensor unit 20, not to implement the dummy heating action during a period of continuously repeating the gas density measurement action. In an action period T1, an implementation frequency of the gas density measurement action using the sensor unit 20 is higher than that of the gas density measurement action using the sensor unit 10.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a gas sensor, and more particularly to a gas sensor that measures the concentration of a target gas by heating a temperature-sensitive element such as a thermistor. [Background technology]

[0002] Patent Document 1 discloses a gas sensor that measures the concentration of a target gas by heating a detection thermistor and a reference thermistor to different temperatures. The gas sensor described in Patent Document 1 provides a dummy heating period after a measurement operation, and matches the heating temperature of the detection thermistor during the measurement period with the heating temperature of the reference thermistor during the dummy heating period, thereby reducing the difference in thermal history between the detection thermistor and the reference thermistor. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. WO2020 / 031517 Summary of the Invention [Problem to be solved by the invention]

[0004] However, providing a dummy heating period lengthens the measurement cycle, and so if the concentration of the gas to be detected fluctuates significantly in a short period of time, the measurement results may not be able to keep up with the concentration change.

[0005] This disclosure describes a technology for a gas sensor that measures the concentration of a target gas by heating a temperature-sensitive element such as a thermistor, which allows the measurement results to follow changes in concentration even when the concentration of the target gas fluctuates significantly over a short period of time. [Means for solving the problem]

[0006] A gas sensor according to one aspect of the present disclosure includes: a first sensor unit including first and second temperature sensitive elements connected in series, a first heater for heating the first temperature sensitive element, and a second heater for heating the second temperature sensitive element, and outputting a first detection signal from a connection point between the first temperature sensitive element and the second temperature sensitive element; a second sensor unit including third and fourth temperature sensitive elements connected in series, a third heater for heating the third temperature sensitive element, and a fourth heater for heating the fourth temperature sensitive element, and outputting a second detection signal from a connection point between the third temperature sensitive element and the fourth temperature sensitive element; and a signal processing circuit that controls the first and second sensor units and calculates a concentration of a gas to be detected based on the first and second detection signals, and when controlling the first sensor unit, the signal processing circuit controls the first temperature sensitive element to be within a first temperature range by the first heater. a first gas concentration measurement operation in which the first temperature sensing element is heated to the first temperature range by the third heater and the second temperature sensing element is heated to the second temperature range by the second heater, and a dummy heating operation in which the first temperature sensing element is heated to the second temperature range by the first heater and the second temperature sensing element is heated to the first temperature range by the second heater; and when controlling the second sensor unit, a second gas concentration measurement operation in which the third temperature sensing element is heated to the first temperature range by the third heater and the fourth temperature sensing element is heated to the second temperature range by the fourth heater is repeatedly performed without performing a heating operation in which the third temperature sensing element is heated to the second temperature range by the third heater and the fourth temperature sensing element is heated to the first temperature range by the fourth heater; and during the first operation period, the second gas concentration measurement operation is performed more frequently than the first gas concentration measurement operation. [Effects of the Invention]

[0007] According to the present disclosure, in a gas sensor that measures the concentration of a target gas by heating a temperature-sensitive element such as a thermistor, a technology is provided that enables measurement results to follow changes in concentration even when the concentration of the target gas fluctuates significantly over a short period of time. [Brief explanation of the drawings]

[0008] [Figure 1]FIG. 1 is a circuit diagram showing a configuration of a gas sensor 100 according to an embodiment of the technology disclosed herein. [Figure 2] 2(a) and 2(b) are schematic diagrams for explaining an example of the gas concentration measurement operation by the sensor units 10 and 20. FIG. [Figure 3] FIG. 3 is a flowchart for explaining the operation of the sensor unit 10. [Figure 4] FIG. 4 is a timing chart for explaining the operation of the sensor unit 10. As shown in FIG. [Figure 5] FIG. 5 is a graph showing the relationship between the ratio of the off period Toff2 to the on period Ton2 (Toff2 / Ton2) and the drift amount per unit time of the measurement results of the CO2 gas concentration. [Figure 6] FIG. 6 is a graph showing the change in the output signal Vout in an atmosphere in which the CO2 gas concentration is controlled. [Figure 7] FIG. 7 is a timing chart for explaining the operation of the modified example of the sensor unit 10. In FIG. [Figure 8] FIG. 8 is a flowchart for explaining the operation of the sensor unit 20. [Figure 9] FIG. 9 is a timing chart for explaining the operation of the sensor unit 20. As shown in FIG. [Figure 10] FIG. 10 is a timing chart for explaining the operation of the sensor units 10 and 20 during the operation period T1. [Figure 11] FIG. 11 is a timing chart for explaining how the gas concentration measurement operation using the sensor unit 20 is intermittently performed. [Figure 12] FIG. 12 is a flowchart for explaining the operation of the sensor unit 20 according to the modified example. [Figure 13] FIG. 13 is a timing chart for explaining an example of the operation of the modified sensor unit 20. In FIG. [Figure 14] FIG. 14 is a timing chart for explaining another example of the operation of the modified sensor unit 20. In FIG. 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] FIG. 1 is a circuit diagram showing a configuration of a gas sensor 100 according to an embodiment of the technology disclosed herein.

[0011] 1, the gas sensor 100 according to this embodiment includes two sensor units 10 and 20 that detect the concentration of a gas to be detected, a temperature sensor 30, and a signal processing circuit 40. Although not particularly limited, the gas sensor 100 according to this embodiment is a thermal conduction type gas sensor for detecting the concentration of CO2 gas in a measurement atmosphere.

[0012] The sensor unit 10 includes thermistors Rd2 and Rd1 connected in series between a power supply Vcc and ground GND, and heaters MH1 and MH2 for heating thermistors Rd1 and Rd2, respectively. The detection signal Vgas1 from the sensor unit 10 appears at the connection point N1 between thermistors Rd1 and Rd2. Thermistor Rd2 is a temperature-sensing element for detection, and thermistor Rd1 is a reference temperature-sensing element. Thermistors Rd1 and Rd2 are resistors whose resistance changes with temperature. Examples of materials for thermistors Rd1 and Rd2 and thermistors Rd3, Rd4, and Rd5 (described later) include vanadium oxide, amorphous silicon, polycrystalline silicon, oxides with a spinel crystal structure containing manganese, titanium oxide, and yttrium-barium-copper oxide. During gas concentration measurement, the thermistor Rd1 is heated by the heater MH1 to, for example, about 300°C (an example of a first temperature range), and the thermistor Rd2 is heated by the heater MH2 to, for example, about 150°C (an example of a second temperature range). The first temperature range is, for example, a predetermined temperature range within the range of 250°C to 450°C, for example, a temperature range around 300°C. The second temperature range is, for example, a predetermined temperature range within the range of 100°C to 230°C, for example, a temperature range around 150°C. In this specification, a "temperature range" has a temperature width of, for example, 1°C or less. For example, a temperature range around 150°C may be a range of 149.5°C to 150.5°C. Furthermore, a temperature range around 300°C may be a range of 299.5°C to 300.5°C. Thermistor Rd1 is designed to have a predetermined resistance value when heated to 300° C., while thermistor Rd2 is designed to have a predetermined resistance value when heated to 150° C. The first temperature range (around 300° C. in this example) is different from the second temperature range (around 150° C. in this example), and in this example, the first temperature range is higher than the second temperature range.

[0013] When the thermistor Rd2, a temperature-sensing element used for detection, is heated to around 150°C and CO2 gas is present in the measurement atmosphere, the heat dissipation characteristics of the thermistor Rd2 change depending on the concentration of CO2 gas. This change manifests itself as a change in the temperature of the thermistor Rd2, i.e., a change in the resistance value of the thermistor Rd2. Specifically, because CO2 gas has a lower heat dissipation rate than air, the temperature of the thermistor Rd2 increases as the CO2 gas concentration increases. Therefore, if the thermistor Rd2 is heated to 150°C when the CO2 gas concentration in the measurement atmosphere is zero, for example, and CO2 gas is present in the measurement atmosphere, the temperature of the thermistor Rd2 will rise above 150°C depending on the concentration of CO2 gas. As a result, the resistance value of the thermistor Rd2 decreases as the CO2 gas concentration in the measurement atmosphere increases.

[0014] On the other hand, even if CO2 gas is present in the measurement atmosphere when the reference temperature sensor thermistor Rd1 is heated to around 300°C, the heat dissipation characteristics of thermistor Rd1 change very little depending on the CO2 gas concentration, and the temperature of thermistor Rd1 also changes very little. Therefore, the change in resistance of thermistor Rd1 heated to around 300°C due to the CO2 gas concentration is much smaller than the change in resistance of thermistor Rd2 heated to around 150°C due to the CO2 gas concentration. It is not necessary for the resistance of thermistor Rd1 heated to around 300°C to change very little due to the CO2 gas concentration. As a result, when the thermistor Rd2 is heated to approximately 150°C and the thermistor Rd1 is heated to approximately 300°C (for example, when the CO2 gas concentration in the measurement atmosphere is zero, the thermistor Rd1 is heated to 300°C and the thermistor Rd2 is heated to 150°C), a detection signal Vgas1 corresponding to the CO2 gas concentration in the measurement atmosphere appears at the connection point N1 between the thermistors Rd1 and Rd2. On the other hand, even if the measurement atmosphere contains another gas whose heat dissipation characteristics are not significantly different between when the thermistor Rd2 is heated to approximately 150°C and when the thermistor Rd1 is heated to approximately 300°C, the concentration of that gas has almost no effect on the detection signal Vgas1. This enables the sensor unit 10 to selectively detect the concentration of CO2 gas.

[0015] The sensor unit 20 has the same circuit configuration as the sensor unit 10. That is, the sensor unit 20 includes thermistors Rd4 and Rd3 connected in series in this order between a power supply Vcc and ground GND, and heaters MH3 and MH4 that heat the thermistors Rd3 and Rd4, respectively. The detection signal Vgas2 of the sensor unit 20 appears at the connection point N2 between the thermistors Rd3 and Rd4. The thermistor Rd4 is a temperature-sensing element for detection and may have the same configuration as the thermistor Rd2 included in the sensor unit 10. The thermistor Rd3 is a temperature-sensing element for reference and may have the same configuration as the thermistor Rd1 included in the sensor unit 10. The thermistors Rd3 and Rd4 are resistors whose resistance changes with temperature. During gas concentration measurement operation, the thermistor Rd3 is heated by the heater MH3 to, for example, about 300°C (an example of a first temperature range), and thermistor Rd4 is heated by the heater MH4 to, for example, about 150°C (an example of a second temperature range). The thermistor Rd3, like the thermistor Rd1 included in the sensor unit 10, is designed to have a predetermined resistance value when heated to 300°C, while the thermistor Rd4, like the thermistor Rd2 included in the sensor unit 10, is designed to have a predetermined resistance value when heated to 150°C.

[0016] The temperature sensor 30 includes a thermistor Rd5 and a fixed resistor R1 connected in series between a power supply Vcc and ground GND. The temperature detection signal Vtemp of the temperature sensor 30 appears at a connection point N3 between the thermistor Rd5 and the fixed resistor R1. The temperature sensor 30 detects the ambient temperature, which is the temperature of the atmosphere being measured. The temperature sensor 30 may be designed to be unaffected or less susceptible to heating by heaters MH1, MH2, MH3, and MH4, for example.

[0017] The signal processing circuit 40 includes differential amplifiers 41 and 42, a buffer 43, an AD converter (ADC) 44, a DA converter (DAC) 45, and a control circuit 46.

[0018] The differential amplifier 41 compares the detection signal Vgas1 with the reference signal Vref to generate an amplified signal Vamp1 by amplifying the level difference between the detection signal Vgas1 and the reference signal Vref (=Vgas1-Vref). The differential amplifier 42 compares the detection signal Vgas2 with the reference signal Vref to generate an amplified signal Vamp2 by amplifying the level difference between the detection signal Vgas2 and the reference signal Vref (=Vgas2-Vref). The buffer 43 buffers the temperature detection signal Vtemp to generate an amplified signal Vamp3. The amplified signals Vamp1 to Vamp3 are input to an AD converter 44. The AD converter 44 performs AD conversion on the amplified signals Vamp1 to Vamp3 to generate digital values ​​and supplies them to a control circuit 46.

[0019] The control circuit 46 calculates the concentration of CO2 gas, which is the gas to be detected, based on the AD-converted amplified signal Vamp1 or Vamp2, and generates an output signal Vout indicating the concentration of CO2 gas. A calculation formula set in the control circuit 46 is used to calculate the concentration of CO2 gas. Furthermore, the control circuit 46 supplies digital values ​​of various control parameters to a DA converter 45. The DA converter 45 converts the digital values ​​of the various control parameters into analog values ​​to generate heater voltages Vmh1 to Vmh4 and a reference signal Vref. The heater voltages Vmh1 to Vmh4 are applied to heaters MH1 to MH4, respectively, thereby heating thermistors Rd1 to Rd4. Furthermore, the reference signal Vref is supplied to differential amplifiers 41 and 42.

[0020] The control circuit 46 corrects the heater voltages Vmh1 to Vmh4 in accordance with the AD converted amplified signal Vamp3. That is, regardless of the ambient temperature, when the CO2 gas concentration in the measurement atmosphere is, for example, zero, the control circuit 46 corrects the heater voltages Vmh1 to Vmh4 so that the temperatures of thermistors Rd2 and Rd4 are both 150°C and the temperatures of thermistors Rd1 and Rd3 are both 300°C.

[0021] Next, the operation of the gas sensor 100 according to this embodiment will be described.

[0022] The gas sensor 100 according to this embodiment generates both the output signal Vout using the detection signal Vgas1 of the sensor unit 10 and the output signal Vout using the detection signal Vgas2 of the sensor unit 20. The signal processing circuit 40 controls the sensor unit 10 to suppress changes over time, while it also controls the sensor unit 20 to correctly detect the concentration of the target gas even when the concentration of the target gas fluctuates significantly in a short period of time.

[0023] 2(a) and 2(b) are schematic diagrams for explaining an example of the gas concentration measurement operation by the sensor units 10 and 20. FIG.

[0024] 2(a), in operation period T1, the gas concentration measurement operation by sensor unit 10 and the gas concentration measurement operation by sensor unit 20 are performed in parallel, and in operation period T2, the gas concentration measurement operation by sensor unit 10 is performed while the gas concentration measurement operation by sensor unit 20 is stopped. Operation period T1 and operation period T2 are separate and do not overlap. Operation period T1 may be shorter than operation period T2.

[0025] 2(b), during operation period T2, the sensor unit 10 performs a gas concentration measurement operation while the sensor unit 20 stops the gas concentration measurement operation, and during operation period T3, the sensor unit 20 performs a gas concentration measurement operation while the sensor unit 10 stops the gas concentration measurement operation. The operation periods T3 and T2 are separate and do not overlap. The operation period T3 may be shorter than the operation period T2.

[0026] Fig. 3 is a flowchart for explaining the operation of the sensor unit 10. Fig. 4 is a timing chart for explaining the operation of the sensor unit 10.

[0027] When performing a gas concentration measurement operation using the sensor unit 10, first, the signal processing circuit 40 included in the gas sensor 100 samples the temperature detection signal Vtemp and calculates the ambient temperature (step 101). The temperature detection signal Vtemp is sampled at timing t10 ​​shown in Fig. 4. Timing t10 ​​is immediately before timing t1 when the heaters MH1 and MH2 start heating the thermistors Rd1 and Rd2.

[0028] Next, the control circuit 46 included in the signal processing circuit 40 outputs a heater instruction value calculated based on the ambient temperature to the DA converter 45, thereby starting heating of the thermistors Rd1 and Rd2 (step 102). The heater instruction value is converted by the DA converter 45 into heater voltages Vmh1 and Vmh2, which are applied to the heaters MH1 and MH2, respectively. In step 102, the thermistor Rd1 is heated to approximately 300°C, and thermistor Rd2 is heated to approximately 150°C. Heating of the thermistors Rd1 and Rd2 starts at timing t1 shown in FIG. 3 (when the CO2 gas concentration in the measurement atmosphere is zero, for example, the thermistor Rd1 is heated to 300°C, and thermistor Rd2 is heated to 150°C).

[0029] Because the temperatures of the thermistors Rd1 and Rd2 do not stabilize until a predetermined time has elapsed from timing t1 when heating starts, a predetermined standby time is required after heating starts and before sampling of the detection signal Vgas1. Then, at timing t20 when the predetermined standby time has elapsed, the signal processing circuit 40 samples the detection signal Vgas1 (step 103). Then, the signal processing circuit 40 calculates the output signal Vout from the detection signal Vgas1 and outputs the output signal Vout to the outside.

[0030] Next, the control circuit 46 resets the heater instruction value to stop heating the thermistors Rd1 and Rd2 (step 104). Heating of the thermistors Rd1 and Rd2 is stopped at timing t2 shown in FIG. 4. This completes the gas concentration measurement operation using the sensor unit 10. In the gas concentration measurement operation using the sensor unit 10, the period from timing t1 when heating of the thermistors Rd1 and Rd2 begins to timing t2 when heating of thermistors Rd1 and Rd2 is stopped is defined as an on-period Ton1.

[0031] After a predetermined off-period Toff1 has elapsed since timing t2, the control circuit 46 outputs a heater instruction value calculated based on the ambient temperature to the DA converter 45, thereby starting dummy heating of the thermistors Rd1 and Rd2 (step 105). In step 105, the thermistor Rd1 is heated to approximately 150°C (an example of the second temperature range), and thermistor Rd2 is heated to approximately 300°C (an example of the first temperature range). Heating of the thermistors Rd1 and Rd2 begins at timing t3 shown in FIG. 4. Therefore, the off-period Toff1 is defined as the period from timing t2, when heating of the thermistors Rd1 and Rd2 ends, to timing t3, when heating of the thermistors Rd1 and Rd2 begins again.

[0032] After a predetermined on-period Ton2 has elapsed since timing t3, the control circuit 46 resets the heater instruction value to stop the heating of the thermistors Rd1 and Rd2 (step 106). The heating of the thermistors Rd1 and Rd2 is stopped at timing t4 shown in FIG. 4. This completes the dummy heating operation. In the dummy heating operation, the period from timing t3 when the heating of the thermistors Rd1 and Rd2 begins to timing t4 when the heating of the thermistors Rd1 and Rd2 stops is defined as the on-period Ton2.

[0033] After a predetermined off-period Toff2 has elapsed since timing t4, the control circuit 46 resumes the gas concentration measurement operation. That is, the signal processing circuit 40 samples the temperature detection signal Vtemp and calculates the ambient temperature (step 101), and the control circuit 46 outputs a heater indication value calculated based on the ambient temperature, thereby starting heating of the thermistors Rd1 and Rd2 (step 102). Heating of the thermistors Rd1 and Rd2 starts at timing t5 shown in FIG. 4. Therefore, the off-period Toff2 is defined as the interval from timing t4, when heating of the thermistors Rd1 and Rd2 ends, to timing t5, when heating of the thermistors Rd1 and Rd2 starts again.

[0034] By repeatedly performing such an operation at a predetermined cycle, the concentration of the target gas contained in the environment can be periodically detected. Moreover, during the on-period Ton1 in which the gas concentration measurement operation is performed, thermistor Rd1 is heated to approximately 300°C and thermistor Rd2 is heated to approximately 150°C, whereas during the on-period Ton2 in which the dummy heating operation is performed, thermistor Rd1 is heated to approximately 150°C and thermistor Rd2 is heated to approximately 300°C. This reduces the difference in thermal history between thermistor Rd1 and thermistor Rd2, thereby suppressing deterioration over time of sensor unit 10 due to the difference in thermal history. To further reduce the difference in thermal history, it is desirable to make the length of on-period Ton1 and the length of on-period Ton2 the same.

[0035] On the other hand, the lengths of the off periods Toff1 and Toff2 do not need to be the same; the off period Toff2 may be longer than the off period Toff1. This is because the next gas concentration measurement operation is performed immediately after the off period Toff2, and ensuring that the off period Toff2 is long enough reduces measurement errors caused by the effects of residual heat. The effects of residual heat appear, for example, as a drift over time in the value of the output signal Vout.

[0036] Figure 5 is a graph showing the relationship between the ratio of the off period Toff2 to the on period Ton2 (Toff2 / Ton2) and the drift per unit time of the measurement results of the CO2 gas concentration obtained from the output signal Vout in a measurement atmosphere where the CO2 gas concentration is kept constant at 400 ppm.

[0037] As shown in Figure 5, the drift per unit time decreases as the ratio of the off period Toff2 to the on period Ton2 (Toff2 / Ton2) increases. The drift per unit time is saturated when the ratio of the off period Toff2 to the on period Ton2 is 10 or greater, and is nearly zero when the ratio of the on period Ton2 to the off period Toff2 is 20 or greater. Taking this into consideration, in order to sufficiently suppress drift in the CO2 gas concentration measurement results due to the influence of residual heat, it is preferable to set the off period Toff2 to 10 times or greater than the on period Ton2, and more preferably to set it to 20 times or greater. While there is no upper limit to the ratio of the off period Toff2 to the on period Ton2, if the off period Toff2 is too long, the cycle in which the output signal Vout can be obtained becomes longer. Therefore, the length of the off period Toff2 can be set according to the purpose.

[0038] Figure 6 is a graph showing the change in output signal Vout (CO2 gas concentration measurement result obtained from output signal Vout) in an atmosphere with a controlled CO2 gas concentration. The solid line shows the case where the ratio of the off period Toff2 to the on period Ton2 (Toff2 / Ton2) is 21.5, and the dashed line shows the case where the ratio of the off period Toff2 to the on period Ton2 (Toff2 / Ton2) is 3.63. The CO2 gas concentration is changed in steps from 400 ppm as the base value to 1000 ppm, 2000 ppm, 3000 ppm, 4000 ppm, and 5000 ppm. The ratio of the off period Toff1 to the on period Ton1 (Toff1 / Ton1) is set to a constant ((Toff1 / Ton1) = 5).

[0039] As shown in Figure 6, when the value of Toff2 / Ton2 is 21.5, the output signal OUT shows an accurate value, whereas when the value of Toff2 / Ton2 is 3.63, the drift that occurs in the output signal OUT increases over time.

[0040] FIG. 7 is a timing chart for explaining the operation of the modified example of the sensor unit 10. In FIG.

[0041] As shown in FIG. 7, in the operation of the modified example of sensor unit 10, the off period Toff11 of thermistor Rd1 (the interval between the operation of heating thermistor Rd1 to around 300°C (first temperature range) by heater MH1 in the gas concentration measurement operation and the operation of heating thermistor Rd1 to around 150°C (second temperature range) by heater MH1 in the dummy heating operation) is set longer than the off period Toff12 of thermistor Rd2 (the interval between the operation of heating thermistor Rd2 to around 150°C (second temperature range) by heater MH2 in the gas concentration measurement operation and the operation of heating thermistor Rd2 to around 300°C (first temperature range) by heater MH2 in the dummy heating operation). In the example shown in FIG. 7, the off period Toff11 of thermistor Rd1 ends at timing t4. This reduces the difference in temperature between thermistors Rd1 and Rd2 at the timing when the dummy heating operation is started, thereby further reducing the difference in thermal history between thermistors Rd1 and Rd2. In this way, in the dummy heating operation, it is not necessary to heat thermistors Rd1 and Rd2 simultaneously.

[0042] 8 is a flowchart for explaining the operation of the sensor unit 20. FIG. 9 is a timing chart for explaining the operation of the sensor unit 20.

[0043] When performing a gas concentration measurement operation using the sensor unit 20, first, the signal processing circuit 40 included in the gas sensor 100 samples the temperature detection signal Vtemp and calculates the ambient temperature (step 201). The temperature detection signal Vtemp is sampled at timing t30 shown in Fig. 9. Timing t30 is immediately before timing t6 when heaters MH3 and MH4 start heating the thermistors Rd3 and Rd4.

[0044] Next, the control circuit 46 included in the signal processing circuit 40 outputs a heater instruction value calculated based on the ambient temperature to the DA converter 45, thereby starting heating of the thermistors Rd3 and Rd4 (step 202). The heater instruction value is converted by the DA converter 45 into heater voltages Vmh3 and Vmh4, which are applied to the heaters MH3 and MH4, respectively. In step 202, the thermistor Rd3 is heated to approximately 300°C, and the thermistor Rd4 is heated to approximately 150°C (when the CO2 gas concentration in the measurement atmosphere is zero, for example, the thermistor Rd3 is heated to 300°C, and thermistor Rd4 is heated to 150°C). Heating of the thermistors Rd3 and Rd4 starts at timing t6 shown in FIG. 9.

[0045] Because the temperatures of the thermistors Rd3 and Rd4 do not stabilize until a predetermined time has elapsed from timing t6 when heating starts, a predetermined standby time is required after heating starts and before sampling of the detection signal Vgas2. Then, at timing t40 when the predetermined standby time has elapsed, the signal processing circuit 40 samples the detection signal Vgas2 (step 203). Then, the signal processing circuit 40 calculates the output signal Vout from the detection signal Vgas2 and outputs the output signal Vout to the outside.

[0046] Next, the control circuit 46 resets the heater instruction value to stop heating the thermistors Rd3 and Rd4 (step 204). Heating of the thermistors Rd3 and Rd4 is stopped at timing t7 shown in FIG. 9. This completes the gas concentration measurement operation using the sensor unit 20. In the gas concentration measurement operation using the sensor unit 20, the period from timing t6 when heating of the thermistors Rd3 and Rd4 begins to timing t7 when heating of the thermistors Rd3 and Rd4 stops is defined as an on-period Ton3. The on-period Ton3 may be the same as the on-period Ton1 in the sensor unit 10.

[0047] After a predetermined off-period Toff3 has elapsed since timing t7, the control circuit 46 resumes the gas concentration measurement operation. Specifically, the signal processing circuit 40 samples the temperature detection signal Vtemp and calculates the ambient temperature (step 201). The control circuit 46 then outputs a heater indicator value calculated based on the ambient temperature, thereby starting heating of the thermistors Rd3 and Rd4 (step 202). Heating of the thermistors Rd3 and Rd4 begins at timing t8 shown in FIG. 9 . Therefore, the off-period Toff3 is defined as the interval between timing t7, when heating of the thermistors Rd3 and Rd4 ends, and timing t8, when heating of the thermistors Rd3 and Rd4 resumes. The off-period Toff3 may be shorter than the off-period Toff2 of the sensor unit 10. The off-period Toff3 may be the same as the off-period Toff1 of the sensor unit 10, or may be shorter than the off-period Toff1 of the sensor unit 10.

[0048] By repeatedly performing such an operation at a predetermined cycle, the concentration of the target gas contained in the environment can be detected periodically.

[0049] In this way, in the sensor unit 10, the gas concentration measurement operation and the dummy heating operation are performed alternately, whereas in the sensor unit 20, the gas concentration measurement operation is performed continuously with an off period Toff3 in between, without any intervening dummy heating operation (a heating operation in which thermistor Rd3 is heated to a temperature range (second temperature range) around 150°C for a predetermined period by heater MH3, and thermistor Rd4 is heated to a temperature range (first temperature range) around 300°C for a predetermined period by heater MH4).

[0050] FIG. 10 is a timing chart for explaining the operation of the sensor units 10 and 20 during the operation period T1 shown in FIG.

[0051] As shown in FIG. 10 , during operation period T1, the gas concentration measurement operation in sensor unit 20 is executed more frequently than the gas concentration measurement operation in sensor unit 10. An execution cycle C1 of the gas concentration measurement operation in sensor unit 10 is defined by the period from time t1 to time t5. An execution cycle C2 of the gas concentration measurement operation in sensor unit 20 is defined by the period from time t6 to time t8. The execution cycle C2 is shorter than the execution cycle C1, and as a result, during operation period T1, the number of times the gas concentration measurement operation in sensor unit 20 is executed per unit time is greater than the number of times the gas concentration measurement operation in sensor unit 10 is executed per unit time. The execution cycle C2 can be shortened because, as described above, no dummy heating operation is performed on sensor unit 20 while the gas concentration measurement operation is being continuously executed. Furthermore, during the operating period T3 shown in Figure 2(b), the operation of the sensor unit 10 is stopped, so naturally the gas concentration measurement operation in the sensor unit 20 is performed more frequently than the gas concentration measurement operation in the sensor unit 10, and the number of times the gas concentration measurement operation in the sensor unit 20 is performed per unit time is greater than the number of times the gas concentration measurement operation in the sensor unit 10 is performed per unit time.

[0052] As a result, during operation period T1, stable gas concentration measurement with little change over time can be performed using sensor unit 10, and even if the CO2 gas concentration fluctuates significantly in a short period of time, the output signal Vout can follow the concentration change due to frequent gas concentration measurement using sensor unit 20. Furthermore, during operation period T2, stable gas concentration measurement with little change over time can be performed using sensor unit 10, and during operation period T3, stable gas concentration measurement with little change over time can be performed due to frequent gas concentration measurement using sensor unit 20, and even if the CO2 gas concentration fluctuates significantly in a short period of time, the output signal Vout can follow the concentration change.

[0053] The gas concentration measurement operation using sensor unit 10 and the gas concentration measurement operation using sensor unit 20 may be performed asynchronously or may be performed with a timing staggered relative to each other. For example, if the timing of the gas concentration measurement operations of sensor unit 10 and sensor unit 20 is staggered so that time t20 shown in Fig. 4 and time t40 shown in Fig. 9 do not overlap, the amplified signals Vamp1 and Vamp2 will not be input to AD converter 44 simultaneously.

[0054] As described with reference to Fig. 2, the gas concentration measurement operation using the sensor unit 20 does not need to be performed constantly, but may be performed intermittently. For example, as shown in Fig. 11, the gas concentration measurement operation using the sensor unit 20 may be started in response to a start signal S supplied from an external device, and may be stopped in response to a stop signal E supplied from the external device. The start signal S and the stop signal E are supplied to the control circuit 46, as shown in Fig. 1. Alternatively, a timer or the like may be provided within the signal processing circuit 40, so that the gas concentration measurement operation using the sensor unit 20 is automatically started at a predetermined time, and the gas concentration measurement operation using the sensor unit 20 is automatically stopped after the predetermined time has elapsed.

[0055] As described above, since the dummy heating operation is not performed on the sensor unit 20 while the gas concentration measurement operation is continuously performed, a difference in thermal history occurs between thermistors Rd3 and Rd4, which causes the sensor unit 20 to change over time. For example, if the cumulative execution time of the gas concentration measurement operation of the sensor unit 20 during the operation periods T1 and T2 is controlled to be shorter than the cumulative execution time of the gas concentration measurement operation of the sensor unit 10, the change over time of the sensor unit 20 can be suppressed. Although the gas concentration measurement operation using the sensor unit 20 is stopped during the operation period T2, the gas concentration measurement operation using the sensor unit 10 and the dummy heating operation are periodically repeated, so that the output signal Vout can be periodically obtained. To suppress the change over time of the sensor unit 20, the operation period T1 or T3 may be shorter than the operation period T2 shown in FIG. 2.

[0056] Alternatively, during the operation period T1, the value of the output signal Vout calculated from the detection signal Vgas1 may be compared with the value of the output signal Vout calculated from the detection signal Vgas2. If a non-negligible difference is found between the two, it may be determined that drift in the output signal Vout occurs due to residual heat in the sensor unit 20 caused by a short execution cycle C2, and the operation period T2 may be extended so that the difference between the two becomes negligible. Alternatively, if it is determined that the difference in thermal history between the thermistors Rd3 and Rd4 has increased and that the sensor unit 20 has significantly changed over time, the gas concentration measurement operation using the sensor unit 20 may be prohibited. Furthermore, the drift component and the change-over-time component may be removed by correcting the calculation formula for calculating the output signal Vout from the detection signal Vgas2 based on the difference between the value of the output signal Vout calculated from the detection signal Vgas1 and the value of the output signal Vout calculated from the detection signal Vgas2.

[0057] FIG. 12 is a flowchart for explaining the operation of the sensor unit 20 according to the modified example.

[0058] In the modification shown in FIG. 12, the sensor unit 20 not only performs a gas concentration measurement operation but also a dummy heating operation. When performing a gas concentration measurement operation using the sensor unit 20, first, the signal processing circuit 40 included in the gas sensor 100 samples the temperature detection signal Vtemp and calculates the ambient temperature (step 301). Next, the control circuit 46 included in the signal processing circuit 40 outputs a heater instruction value calculated based on the ambient temperature to the DA converter 45, thereby starting heating of the thermistors Rd3 and Rd4 (step 302). The heater instruction values ​​are converted by the DA converter 45 into heater voltages Vmh3 and Vmh4, which are applied to the heaters MH3 and MH4, respectively. In step 302, the thermistor Rd3 is heated to approximately 300°C, and the thermistor Rd4 is heated to approximately 150°C (when the CO2 gas concentration in the measurement atmosphere is zero, for example, the thermistor Rd3 is heated to 300°C, and the thermistor Rd4 is heated to 150°C).

[0059] The signal processing circuit 40 samples the detection signal Vgas2 when a predetermined standby time has elapsed (step 303). Then, the signal processing circuit 40 calculates the output signal Vout from the detection signal Vgas2 and outputs the output signal Vout to the outside. Next, the control circuit 46 stops heating the thermistors Rd3 and Rd4 by resetting the heater indication value (step 304). In the gas concentration measurement operation using the sensor unit 20, the period from when heating of the thermistors Rd3 and Rd4 is started in step 302 to when heating of thermistors Rd3 and Rd4 is stopped in step 304 is defined as an on-period Ton3.

[0060] Next, the control circuit 46 determines whether to stop the gas concentration measurement operation using the sensor unit 20 (step 305). Whether to stop the gas concentration measurement operation using the sensor unit 20 may be determined based on whether a stop signal E has been input. If it is determined not to stop the gas concentration measurement operation using the sensor unit 20, the process returns to step 301, where the ambient temperature is calculated, and heating of the thermistors Rd3 and Rd4 is restarted. In the gas concentration measurement operation using the sensor unit 20, the period from when heating of the thermistors Rd3 and Rd4 is stopped to when heating is restarted is defined as the off period Toff3. On the other hand, if it is determined to stop the gas concentration measurement operation using the sensor unit 20, the control circuit 46 stores the total heating time of the thermistors Rd3 and Rd4 during the gas concentration measurement operation using the sensor unit 20 (total time of the on periods Ton3) (step 306), stops the gas concentration measurement operation using the sensor unit 20, and starts a dummy heating operation for the sensor unit 20 (step 307).

[0061] In the dummy heating operation for the sensor unit 20, first, dummy heating of the thermistors Rd3 and Rd4 is started (step 308). In the dummy heating of the thermistors Rd3 and Rd4, the thermistor Rd3 is heated to approximately 150°C, and the thermistor Rd4 is heated to approximately 300°C. Next, the control circuit 46 stops heating of the thermistors Rd3 and Rd4 (step 309). In the dummy heating operation for the sensor unit 20, the period from when heating of the thermistors Rd3 and Rd4 is started in step 308 to when heating of thermistors Rd3 and Rd4 is stopped in step 309 is defined as an on-period Ton4. The length of the on-period Ton4 may be the same as the length of the on-period Ton3.

[0062] Next, the control circuit 46 determines whether the total heating time of the thermistors Rd3, Rd4 during the dummy heating operation of the sensor unit 20 (the total time of the on-periods Ton4) is less than the total heating time of the thermistors Rd3, Rd4 during the gas concentration measurement operation using the sensor unit 20 (the total time of the on-periods Ton3) (step 310). If the determination result shows that the total time of the on-periods Ton4 is less than the total time of the on-periods Ton3, that is, if the total heating time during the dummy heating operation of the sensor unit 20 is less than the total heating time during the gas concentration measurement operation using the sensor unit 20, the control circuit 46 determines whether to stop the dummy heating operation of the sensor unit 20 (step 311). Whether to stop the dummy heating operation of the sensor unit 20 may also be determined based on whether the start signal S has been input. If it is determined not to stop the dummy heating operation of the sensor unit 20, the process returns to step 308, and dummy heating of the thermistors Rd3 and Rd4 is started again. In the dummy heating operation of the sensor unit 20, the period from when the dummy heating of the thermistors Rd3 and Rd4 is stopped to when the dummy heating is started is defined as the off period Toff4.

[0063] On the other hand, if it is determined in step 310 that the total time of the on period Ton4 is equal to or greater than the total time of the on period Ton3, that is, if it is determined that the total heating time during the dummy heating operation of the sensor unit 20 is equal to or greater than the total heating time during the gas concentration measurement operation using the sensor unit 20, or if it is determined in step 311 that the dummy heating operation of the sensor unit 20 should be stopped, the control circuit 46 stores the total heating time (total time of the on period Ton4) of the thermistors Rd3 and Rd4 during the dummy heating operation of the sensor unit 20 (step 312) and stops the dummy heating operation of the sensor unit 20 (step 313).

[0064] FIG. 13 is a timing chart for explaining an example of the operation of the modified sensor unit 20. In FIG.

[0065] In the example shown in FIG. 13 , after a gas concentration measurement operation using the sensor unit 20 is performed in period 401, a dummy heating operation of the sensor unit 20 is performed in period 402, which exists between the time when a gas concentration measurement operation using the sensor unit 20 is performed again in period 403. Both periods 401 and 403 start in response to a start signal S and end in response to a stop signal E. On the other hand, period 402 may start when an arbitrary standby time has elapsed after the end of period 401. Furthermore, the lengths of the on-periods Ton3 and Ton4 do not need to be the same. As shown in the example shown in FIG. 14 , the length of the on-period Ton4 may be set longer than the length of the on-period Ton3. In this case, step 308 may be continued until the total heating time during the dummy heating operation of the sensor unit 20 (the total time of the on-periods Ton4) reaches the total heating time during the gas concentration measurement operation using the sensor unit 20 (the total time of the on-periods Ton3).

[0066] 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.

[0067] For example, in the above-described embodiments, a thermistor, which is a resistor, is used as the temperature-sensing element of the sensor units 10 and 20, but the present invention is not limited to this. For example, platinum (Pt) or tungsten (W), which is a resistor, may be used as the temperature-sensing element.

[0068] The technology according to the present disclosure includes, but is not limited to, the following configuration examples.

[0069] A gas sensor according to one aspect of the present disclosure includes: a first sensor unit including first and second temperature sensitive elements connected in series, a first heater for heating the first temperature sensitive element, and a second heater for heating the second temperature sensitive element, and outputting a first detection signal from a connection point between the first temperature sensitive element and the second temperature sensitive element; a second sensor unit including third and fourth temperature sensitive elements connected in series, a third heater for heating the third temperature sensitive element, and a fourth heater for heating the fourth temperature sensitive element, and outputting a second detection signal from a connection point between the third temperature sensitive element and the fourth temperature sensitive element; and a signal processing circuit that controls the first and second sensor units and calculates a concentration of a gas to be detected based on the first and second detection signals, and when controlling the first sensor unit, the signal processing circuit controls the first temperature sensitive element to be within a first temperature range by the first heater. a first gas concentration measurement operation in which the first temperature sensing element is heated to the first temperature range by the third heater and the second temperature sensing element is heated to the second temperature range by the second heater, and a dummy heating operation in which the first temperature sensing element is heated to the second temperature range by the first heater and the second temperature sensing element is heated to the first temperature range by the second heater; and when controlling the second sensor unit, a second gas concentration measurement operation in which the third temperature sensing element is heated to the first temperature range by the third heater and the fourth temperature sensing element is heated to the second temperature range by the fourth heater is repeatedly performed without performing a heating operation in which the third temperature sensing element is heated to the second temperature range by the third heater and the fourth temperature sensing element is heated to the first temperature range by the fourth heater; and during the first operation period, the second gas concentration measurement operation is performed more frequently than the first gas concentration measurement operation. According to this, it is possible to perform stable gas concentration measurement with little change over time using the first sensor unit, and to perform gas concentration measurement with high frequency using the second sensor unit.

[0070] In the above gas sensor, the signal processing circuit may execute the first gas concentration measurement operation and the dummy heating operation during the second operation period, and may stop the second gas concentration measurement operation, thereby suppressing deterioration over time of the second sensor portion.

[0071] In the gas sensor described above, the signal processing circuit may periodically execute the first gas concentration measurement operation and the dummy heating operation during the first and second operation periods, thereby periodically calculating the concentration of the target gas.

[0072] In the gas sensor, the cumulative execution time of the second gas concentration measurement operation during the first and second operation periods may be shorter than the cumulative execution time of the first gas concentration measurement operation, thereby suppressing deterioration over time of the second sensor unit.

[0073] In the above gas sensor, the signal processing circuit may perform a second dummy heating operation in the second operation period, in which the third heater heats the third thermosensitive element to the second temperature range and the fourth heater heats the fourth thermosensitive element to the first temperature range, thereby suppressing deterioration over time of the second sensor unit caused by a difference in thermal history between the third thermosensitive element and the fourth thermosensitive element.

[0074] In the above gas sensor, the temperature in the first temperature range may be higher than the second temperature range, and the interval between the operation of heating the first temperature sensing element to the first temperature range by the first heater in the first gas concentration measurement operation and the operation of heating the first temperature sensing element to the second temperature range by the first heater in the dummy heating operation may be longer than the interval between the operation of heating the second temperature sensing element to the second temperature range by the second heater in the first gas concentration measurement operation and the operation of heating the second temperature sensing element to the first temperature range by the second heater in the dummy heating operation. This makes it possible to reduce the thermal history difference between the first and second temperature sensing elements.

[0075] In the gas sensor, the interval between the dummy heating operation and the first gas concentration measurement operation may be 10 times or more the execution time of the dummy heating operation, thereby reducing measurement errors of the first sensor unit caused by residual heat. [Explanation of symbols]

[0076] 10,20 Sensor section 30 Temperature Sensor 40 Signal Processing Circuit 41,42 Differential Amplifier 43 buffers 44 AD converters 45 DA converter 46 Control circuit 100 Gas Sensor 101~106, 201~204, 301~313 steps 401~403 period C1,C2 execution cycle E stop signal MH1~MH4 heaters N1~N3 connection points R1 fixed resistor Rd1~Rd4 Thermistor S start signal T1~T3 operating period Vamp1~Vamp3 amplified signals Vgas1, Vgas2 detection signal Vmh1~Vmh4 heater voltage Vout output signal Vref reference signal Vtemp temperature detection signal

Claims

1. a first sensor unit including first and second thermosensitive elements connected in series, a first heater for heating the first thermosensitive element, and a second heater for heating the second thermosensitive element, and outputting a first detection signal from a connection point between the first thermosensitive element and the second thermosensitive element; a second sensor unit including third and fourth thermosensitive elements connected in series, a third heater for heating the third thermosensitive element, and a fourth heater for heating the fourth thermosensitive element, and outputting a second detection signal from a connection point between the third thermosensitive element and the fourth thermosensitive element; a signal processing circuit that controls the first and second sensor units and calculates the concentration of the target gas based on the first and second detection signals; Equipped with The signal processing circuit When controlling the first sensor unit, a first gas concentration measurement operation is alternately performed in which the first heater heats the first temperature-sensing element to a first temperature range and the second heater heats the second temperature-sensing element to a second temperature range, and a dummy heating operation is alternately performed in which the first heater heats the first temperature-sensing element to the second temperature range and the second heater heats the second temperature-sensing element to the first temperature range, When controlling the second sensor unit, a second gas concentration measurement operation in which the third heater heats the third temperature sensitive element to the first temperature range and the fourth heater heats the fourth temperature sensitive element to the second temperature range is repeatedly performed without performing a heating operation in which the third heater heats the third temperature sensitive element to the second temperature range and the fourth heater heats the fourth temperature sensitive element to the first temperature range, During a first operation period, the second gas concentration measurement operation is performed more frequently than the first gas concentration measurement operation. Gas sensor.

2. the signal processing circuit executes the first gas concentration measurement operation and the dummy heating operation during a second operation period, and stops the second gas concentration measurement operation; 2. The gas sensor according to claim 1.

3. the signal processing circuit periodically executes the first gas concentration measurement operation and the dummy heating operation during the first and second operation periods; 3. The gas sensor according to claim 2.

4. a cumulative execution time of the second gas concentration measurement operation in the first and second operation periods is shorter than a cumulative execution time of the first gas concentration measurement operation; 3. The gas sensor according to claim 2.

5. the signal processing circuit executes a second dummy heating operation in which the third heater heats the third thermosensitive element to the second temperature range and the fourth heater heats the fourth thermosensitive element to the first temperature range during the second operation period; 3. The gas sensor according to claim 2.

6. the temperature in the first temperature range is higher than the temperature in the second temperature range, an interval between an operation of heating the first temperature sensing element to the first temperature range by the first heater in the first gas concentration measurement operation and an operation of heating the first temperature sensing element to the second temperature range by the first heater in the dummy heating operation is longer than an interval between an operation of heating the second temperature sensing element to the second temperature range by the second heater in the first gas concentration measurement operation and an operation of heating the second temperature sensing element to the first temperature range by the second heater in the dummy heating operation; 2. The gas sensor according to claim 1.

7. an interval between the dummy heating operation and the first gas concentration measurement operation is 10 times or more the execution time of the dummy heating operation; The gas sensor according to claim 1 .

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