Gas sensor

Abstract

The purpose of the present invention is to change a measurement result in accordance with the concentration even when the concentration of a gas to be detected drastically fluctuates in a short period of time. A gas sensor (100) is provided with: sensor units (10, 20) that output detection signals (Vgas1, Vgas2), respectively; and a signal processing circuit (40) that calculates the concentration of the gas to be detected on the basis of the detection signals (Vgas1, Vgas2). The signal processing circuit (40) alternately repeats the execution of the gas concentration measurement operation and the dummy heating operation when the sensor unit (10) is controlled, and does not perform the dummy heating operation while the execution of the gas concentration measurement operation is repeated continuously when the sensor unit (20) is controlled. In the operation period (T1), the execution frequency of the gas concentration measurement operation using the sensor unit (20) is higher than the execution frequency of the gas concentration measurement operation using the sensor unit (10).

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 detection target gas by heating a temperature-sensitive element such as a thermistor. Background Art

[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. By aligning the heating temperature of the detection thermistor during the measurement period with the heating temperature of the reference thermistor during the dummy heating period, the heating temperature of the reference thermistor during the measurement period also matches the heating temperature of the detection thermistor during the dummy heating period. This reduces the difference in thermal history between the detection thermistor and the reference thermistor.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: International Publication No. WO2020 / 031517 Summary of the Invention

[0006] The technical problem that the invention aims to solve

[0007] However, providing a dummy heating period lengthens the measurement cycle. Therefore, when the concentration of the detection target gas fluctuates significantly within a short period of time, the measurement result may not be able to follow the concentration change.

[0008] This disclosure describes a technique for ensuring that, in a gas sensor that measures the concentration of a target gas by heating a temperature-sensitive element such as a thermistor, the measurement result follows the concentration change even when the concentration of the target gas fluctuates significantly within a short period of time.

[0009] Means used to solve problems

[0010] According to one aspect of the present disclosure, a gas sensor comprises: a first sensor unit including: first and second temperature-sensing elements connected in series, a first heater for heating the first temperature-sensing element, and a second heater for heating the second temperature-sensing element, and outputting a first detection signal from a connection point between the first temperature-sensing element and the second temperature-sensing element; a second sensor unit including: third and fourth temperature-sensing elements connected in series, a third heater for heating the third temperature-sensing element, and a fourth heater for heating the fourth temperature-sensing element, and outputting a second detection signal from a connection point between the third temperature-sensing element and the fourth temperature-sensing element; and a signal processing circuit that controls the first and second sensor units and calculates the concentration of the detection target gas based on the first and second detection signals, wherein the signal processing circuit alternately repeats a first gas concentration measurement operation and a dummy heating operation when controlling the first sensor unit, wherein the first gas concentration measurement operation is performed by The first heater heats the first temperature sensing element to the first temperature zone, and heats the second temperature sensing element to the second temperature zone through the second heater, and the virtual heating operation heats the first temperature sensing element to the second temperature zone through the first heater, and heats the second temperature sensing element to the first temperature zone through the second heater; when controlling the second sensor part, the signal processing circuit repeatedly performs the second gas concentration measurement operation without performing the heating operation of heating the third temperature sensing element to the second temperature zone through the third heater and heating the fourth temperature sensing element to the first temperature zone through the fourth heater, wherein the second gas concentration measurement operation heats the third temperature sensing element to the first temperature zone through the third heater and heats the fourth temperature sensing element to the second temperature zone through the fourth heater, and during the first operation, the execution frequency of the second gas concentration measurement operation is higher than the execution frequency of the first gas concentration measurement operation.

[0011] Effects of the Invention

[0012] According to the present disclosure, in a gas sensor that measures the concentration of a target gas by heating a temperature sensing element such as a thermistor, a technology is provided that enables the measurement result to follow the concentration change even when the concentration of the target gas fluctuates greatly within a short period of time. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 1 is a circuit diagram showing the structure of a gas sensor 100 according to one embodiment of the technology involved in the present disclosure.

[0014] Figure 2 (a) and (b) are schematic diagrams for explaining an example of the gas concentration measurement operation by the sensor units 10 and 20 .

[0015] Figure 3 This is a flowchart for explaining the operation of the sensor unit 10 .

[0016] Figure 4 It is a timing chart for explaining the operation of the sensor unit 10 .

[0017] Figure 5 This 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 result of the CO2 gas concentration.

[0018] Figure 6 Graph showing changes in the output signal Vout in an atmosphere where the CO 2 gas concentration is controlled.

[0019] Figure 7 It is a timing chart for explaining the operation according to the modified example of the sensor unit 10 .

[0020] Figure 8 This is a flowchart for explaining the operation of the sensor unit 20 .

[0021] Figure 9 It is a timing chart for explaining the operation of the sensor unit 20 .

[0022] Figure 10 This is a timing chart for explaining the operation of the sensor units 10 and 20 in the operation period T1.

[0023] Figure 11 This is a timing chart for explaining a case where the gas concentration measurement operation using the sensor unit 20 is intermittently performed.

[0024] Figure 12 This is a flowchart for explaining the operation according to the modified example of the sensor unit 20 .

[0025] Figure 13 This is a timing chart for explaining an example of the operation according to the modified example of the sensor unit 20 .

[0026] Figure 14 It is a timing chart for explaining another example of the operation according to the modified example of the sensor unit 20 .

[0027] Explanation of symbols

[0028] 10, 20: Sensor part

[0029] 30: Temperature sensor

[0030] 40: Signal processing circuit

[0031] 41, 42: Differential amplifier

[0032] 43: Buffer

[0033] 44: AD converter

[0034] 45: DA converter

[0035] 46: Control circuit

[0036] 100: Gas sensor

[0037] 101-106, 201-204, 301-313: Steps

[0038] 401~403: period

[0039] C1, C2: Execution cycle

[0040] E: Stop signal

[0041] MH1~MH4:Heater

[0042] N1~N3:connection points

[0043] R1: fixed resistor

[0044] Rd1~Rd4:Thermistor

[0045] S: Start signal

[0046] T1~T3: Operation period

[0047] Vamp1~Vamp3: amplify the signal

[0048] Vgas1, Vgas2: detection signal

[0049] Vmh1~Vmh4: heater voltage

[0050] Vout: output signal

[0051] Vref: reference signal

[0052] Vtemp: temperature detection signal DETAILED DESCRIPTION

[0053] Hereinafter, embodiments of the technology according to the present disclosure will be described in detail with reference to the accompanying drawings.

[0054] Figure 1 1 is a circuit diagram showing the structure of a gas sensor 100 according to one embodiment of the technology involved in the present disclosure.

[0055] like Figure 1 As shown, gas sensor 100 according to this embodiment includes two sensor units 10 and 20 for detecting the concentration of a target gas, a temperature sensor 30, and a signal processing circuit 40. Although not particularly limited, gas sensor 100 according to this embodiment is a thermal conduction gas sensor for detecting the concentration of CO2 gas in a measurement atmosphere.

[0056] The sensor unit 10 includes thermistors Rd2 and Rd1 connected in series between a power supply Vcc and a ground line GND, and heaters MH1 and MH2 for heating thermistors Rd1 and Rd2, respectively. A detection signal Vgas1 from the sensor unit 10 appears at a connection point N1 between thermistors Rd1 and Rd2. Thermistor Rd2 is a temperature-sensing element for detection, while thermistor Rd1 is a temperature-sensing element for reference. Thermistors Rd1 and Rd2 are resistors whose resistance value changes with temperature. Examples of materials for thermistors Rd1 and Rd2, as well as thermistors Rd3, Rd4, and Rd5 described later, include vanadium oxide, amorphous silicon, polycrystalline silicon, an oxide having a spinel crystal structure containing manganese, titanium oxide, or yttrium-barium-copper oxide. During gas concentration measurement operation, thermistor Rd1 is heated by heater MH1 to, for example, approximately 300°C (an example of a first temperature range), and thermistor Rd2 is heated by heater MH2 to, for example, approximately 150°C (an example of a second temperature range). The first temperature range is, for example, a predetermined temperature range between 250°C and 450°C, such as a temperature range around 300°C. The second temperature range is, for example, a predetermined temperature range between 100°C and 230°C, such as a temperature range around 150°C. As used herein, a "temperature range" has a temperature range of, for example, 1°C or less. For example, a temperature range around 150°C may refer to a range between 149.5°C and 150.5°C. Alternatively, a temperature range around 300°C may refer to a range between 299.5°C and 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 region (a temperature region of approximately 300° C. in this example) and the second temperature region (a temperature region of approximately 150° C. in this example) are different in temperature. In this example, the temperature of the first temperature region is higher than that of the second temperature region.

[0057] When thermistor Rd2, a temperature-sensing element used for detection, is heated to approximately 150°C and CO2 gas is present in the measurement atmosphere, the heat dissipation characteristics of thermistor Rd2 change depending on its concentration. This change manifests as a change in the temperature of thermistor Rd2, or in other words, a change in the resistance value of thermistor Rd2. Specifically, CO2 gas has lower heat dissipation properties than air. Therefore, the higher the CO2 gas concentration, the higher the temperature of thermistor Rd2. Therefore, if thermistor Rd2 is heated to 150°C while the CO2 gas concentration in the measurement atmosphere is zero, for example, the presence of CO2 gas in the measurement atmosphere will cause the temperature of thermistor Rd2 to rise above 150°C depending on its concentration. As a result, the resistance value of thermistor Rd2 decreases as the CO2 gas concentration in the measurement atmosphere increases.

[0058] On the other hand, even when thermistor Rd1, serving as a reference temperature sensing element, is heated to approximately 300°C and CO₂ gas is present in the measurement atmosphere, the heat dissipation characteristics of thermistor Rd1 barely change depending on the CO₂ gas concentration, and the temperature of thermistor Rd1 barely changes. Therefore, the change in resistance value of thermistor Rd1 due to CO₂ gas concentration when heated to approximately 300°C is significantly smaller than the change in resistance value of thermistor Rd2 due to CO₂ gas concentration when heated to approximately 150°C. The resistance value of thermistor Rd1 also barely changes depending on CO₂ gas concentration when heated to approximately 300°C. As a result, if thermistor Rd2 is heated to approximately 150°C and thermistor Rd1 is heated to approximately 300°C (for example, when the CO2 gas concentration in the measurement atmosphere is zero, if thermistor Rd1 is heated to 300°C and 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 thermistors Rd1 and Rd2. On the other hand, even if the measurement atmosphere contains another gas whose heat dissipation characteristics when thermistor Rd2 is heated to approximately 150°C are not significantly different from those when thermistor Rd1 is heated to approximately 300°C, the concentration of this gas will have little effect on the detection signal Vgas1. Thus, the sensor unit 10 can selectively detect the concentration of CO2 gas.

[0059] The sensor unit 20 has the same circuit structure as the sensor unit 10. That is, the sensor unit 20 includes thermistors Rd4 and Rd3 connected in series between the power supply Vcc and the ground line GND, and heaters MH3 and MH4 for heating thermistors Rd3 and Rd4, respectively. The detection signal Vgas2 of the sensor unit 20 appears at the connection point N2 between thermistors Rd3 and Rd4. Thermistor Rd4 is a temperature-sensing element for detection and may have the same structure as thermistor Rd2 included in the sensor unit 10. Thermistor Rd3 is a temperature-sensing element for reference and may have the same structure as thermistor Rd1 included in the sensor unit 10. Thermistors Rd3 and Rd4 are resistors whose resistance value changes with temperature. During the gas concentration measurement operation, thermistor Rd3 is heated by heater MH3 to, for example, approximately 300°C (an example of the first temperature range), and thermistor Rd4 is heated by heater MH4 to, for example, approximately 150°C (an example of the second temperature range). Thermistor Rd3 is designed to have a predetermined resistance value when heated to 300°C, similar to thermistor Rd1 included in the sensor unit 10. On the other hand, thermistor Rd4 is designed to have a predetermined resistance value when heated to 150°C, similar to thermistor Rd2 included in the sensor unit 10.

[0060] Temperature sensor 30 includes a thermistor Rd5 and a fixed resistor R1 connected in series between a power supply Vcc and a ground line GND. A temperature detection signal Vtemp from temperature sensor 30 appears at a connection point N3 between the thermistor Rd5 and the fixed resistor R1. Temperature sensor 30 detects ambient temperature, which refers to the temperature of the measurement atmosphere. For example, temperature sensor 30 can be designed to be unaffected or less susceptible to heating by heaters MH1, MH2, MH3, and MH4.

[0061] 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 .

[0062] The differential amplifier 41 compares the detection signal Vgas1 with the reference signal Vref to generate an amplified signal Vamp1, which is amplified by 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, which is amplified by 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 the AD converter 44. The AD converter 44 performs A / D conversion on the amplified signals Vamp1 to Vamp3 to generate digital values, which are then supplied to the control circuit 46.

[0063] Based on the A / D-converted amplified signal Vamp1 or Vamp2, the control circuit 46 calculates the concentration of CO2 gas, the target gas, and generates an output signal Vout indicating the CO2 gas concentration. A calculation formula set within the control circuit 46 can be used to calculate the CO2 gas concentration. Furthermore, the control circuit 46 supplies the digital values ​​of various control parameters to the D / A converter 45. The D / A converter 45 performs analog conversion of the digital values ​​of the various control parameters to generate heater voltages Vmh1 to Vmh4 and a reference signal Vref. 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 the differential amplifiers 41 and 42.

[0064] Control circuit 46 corrects heater voltages Vmh1 to Vmh4 based on amplified signal Vamp3 after AD conversion. Specifically, regardless of ambient temperature, when the CO2 gas concentration in the measurement atmosphere is, for example, zero, heater voltages Vmh1 to Vmh4 are corrected 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.

[0065] Next, the operation of the gas sensor 100 according to this embodiment will be described.

[0066] The gas sensor 100 according to this embodiment generates both an output signal Vout using the detection signal Vgas1 from the sensor unit 10 and an output signal Vout using the detection signal Vgas2 from the sensor unit 20. The signal processing circuit 40 controls the sensor unit 10 so as to suppress temporal changes and to control the sensor unit 20 so as to accurately detect the concentration of the target gas even when the concentration of the target gas fluctuates significantly within a short period of time.

[0067] Figure 2 (a) and (b) are schematic diagrams for explaining an example of the gas concentration measurement operation by the sensor units 10 and 20 .

[0068] exist Figure 2 In the example shown in (a), during 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. During 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 different periods and do not overlap. Operation period T1 can also be shorter than operation period T2.

[0069] exist Figure 2 In the example shown in (b), during 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. During operation period T3, the gas concentration measurement operation by sensor unit 20 is performed, while the gas concentration measurement operation by sensor unit 10 is stopped. Operation period T3 and operation period T2 are different periods and do not overlap. Operation period T3 may also be shorter than operation period T2.

[0070] Figure 3 : is a flowchart for explaining the operation of the sensor unit 10. Figure 4 It is a timing chart for explaining the operation of the sensor unit 10 .

[0071] When the gas concentration measurement operation is performed 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). Figure 4 The timing t10 ​​is immediately before the timing t1 when the thermistors Rd1 and Rd2 start to be heated by the heaters MH1 and MH2.

[0072] Next, the control circuit 46 included in the signal processing circuit 40 outputs the heater instruction value calculated based on the ambient temperature to the DA converter 45, thereby starting the heating of the thermistors Rd1 and Rd2 (step 102). The heater instruction value is converted into heater voltages Vmh1 and Vmh2 by the DA converter 45 and applied to the heaters MH1 and MH2 respectively. In step 102, the thermistor Rd1 is heated to approximately 300°C and the thermistor Rd2 is heated to approximately 150°C. The heating of the thermistors Rd1 and Rd2 begins at Figure 3This is performed at the timing t1 shown (when the CO 2 gas concentration in the measurement atmosphere is, for example, zero, the thermistor Rd1 is heated to 300° C., and the thermistor Rd2 is heated to 150° C.).

[0073] The temperatures of thermistors Rd1 and Rd2 remain unstable from the start of heating at time t1 until a predetermined time has passed. Therefore, a predetermined waiting time is required from the start of heating to the sampling of detection signal Vgas1. Then, at time t20 after the predetermined waiting time has passed, signal processing circuit 40 samples detection signal Vgas1 (step 103). Signal processing circuit 40 then calculates output signal Vout from detection signal Vgas1 and outputs output signal Vout to the outside.

[0074] Next, the control circuit 46 stops heating the thermistors Rd1 and Rd2 by resetting the heater instruction value (step 104). Figure 4 The above is performed at the timing t2 shown. The gas concentration measurement operation using the sensor unit 10 is completed. In the gas concentration measurement operation using the sensor unit 10, the period from the timing t1 when heating of thermistors Rd1 and Rd2 is started to the timing t2 when heating of thermistors Rd1 and Rd2 is stopped is defined as the on-period Ton1.

[0075] After a predetermined off-period Toff1 has elapsed from timing t2, the control circuit 46 outputs the heater instruction value calculated based on the ambient temperature to the DA converter 45, thereby starting dummy heating of thermistors Rd1 and Rd2 (step 105). In step 105, 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). The heating of thermistors Rd1 and Rd2 begins at Figure 4 Therefore, the OFF period Toff1 is defined by the period from the timing t2 when the heating of the thermistors Rd1 and Rd2 is ended to the timing t3 when the heating of the thermistors Rd1 and Rd2 is restarted.

[0076] After a predetermined ON period Ton2 has elapsed from timing t3, the control circuit 46 stops heating the thermistors Rd1 and Rd2 by resetting the heater instruction value (step 106). Figure 4 The dummy heating operation is completed as described above. In the dummy heating operation, the period from the timing t3 when heating of thermistors Rd1 and Rd2 is started to the timing t4 when heating of thermistors Rd1 and Rd2 is stopped is defined as the on-period Ton2.

[0077] After the predetermined off-period Toff2 has elapsed from timing t4, 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 101). The control circuit 46 outputs a heater instruction value calculated based on the ambient temperature, thereby starting heating the thermistors Rd1 and Rd2 (step 102). The heating of the thermistors Rd1 and Rd2 begins at Figure 4 Therefore, the off period Toff2 is defined by the interval from the timing t4 at which the heating of the thermistors Rd1 and Rd2 is ended to the timing t5 at which the heating of the thermistors Rd1 and Rd2 is restarted.

[0078] By repeating this operation at a predetermined interval, the concentration of the target gas in the environment can be periodically detected. Furthermore, during the on-period Ton1, during 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. In contrast, during the on-period Ton2, during 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 temporal changes in the sensor unit 10 caused by this difference in thermal history. To further minimize this difference in thermal history, it is desirable that the lengths of the on-period Ton1 and the on-period Ton2 be equal.

[0079] On the other hand, the lengths of the off-periods Toff1 and Toff2 do not need to be the same, and Toff2 can be longer than Toff1. This is because the next gas concentration measurement operation is performed immediately after Toff2. Therefore, ensuring a sufficient length for Toff2 can reduce measurement errors caused by the effects of residual heat. The effects of residual heat manifest themselves, for example, as drift over time in the output signal Vout.

[0080] Figure 5 It 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 CO2 gas concentration measurement result obtained from the output signal Vout in a measurement atmosphere where the CO2 gas concentration is constantly managed at 400 ppm.

[0081] like Figure 5As shown, the longer the ratio of the off-period Toff2 to the on-period Ton2 (Toff2 / Ton2), the lower the drift per unit time. The drift per unit time is approximately saturated when the ratio of the off-period Toff2 to the on-period Ton2 is greater than 10, and is approximately zero when the ratio of the on-period Ton2 to the off-period Toff2 is greater than 20. Taking this into account, it can be seen that in order to fully suppress the drift of the CO2 gas concentration measurement results caused by the influence of residual heat, it is preferable to set the off-period Toff2 to be greater than 10 times the on-period Ton2, and more preferably to be greater than 20 times the on-period Ton2. There is no upper limit to the ratio of the off-period Toff2 to the on-period Ton2, but if the off-period Toff2 is too long, the period of the output signal Vout can be prolonged. Therefore, the length of the off-period Toff2 can be set according to the purpose.

[0082] Figure 6 This graph shows the changes in the output signal Vout (the CO2 gas concentration measurement result obtained from the output signal Vout) in an atmosphere where the CO2 gas concentration is controlled. The solid line shows the case where the ratio of the off-period Toff2 to the on-period Ton2 (Toff2 / Ton2) is set to 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 set to 3.63. The CO2 gas concentration is set at a baseline value of 400 ppm and gradually changed 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 kept constant ((Toff1 / Ton1) = 5).

[0083] like Figure 6 As shown, when the value of Toff2 / Ton2 is 21.5, the output signal Vout shows a correct value. In contrast, when the value of Toff2 / Ton2 is 3.63, it can be seen that the drift generated in the output signal Vout increases with the passage of time.

[0084] Figure 7 It is a timing chart for explaining the operation according to the modified example of the sensor unit 10 .

[0085] like Figure 7As shown, in the operation of the modified example of the sensor unit 10, the off period Toff11 of the thermistor Rd1 (the interval between the operation of heating the thermistor Rd1 to approximately 300°C (the first temperature region) by the heater MH1 in the gas concentration measurement operation and the operation of heating the thermistor Rd1 to approximately 150°C (the second temperature region) by the heater MH1 in the dummy heating operation) is set to be longer than the off period Toff12 of the thermistor Rd2 (the interval between the operation of heating the thermistor Rd2 to approximately 150°C (the second temperature region) by the heater MH2 in the gas concentration measurement operation and the operation of heating the thermistor Rd2 to approximately 300°C (the first temperature region) by the heater MH2 in the dummy heating operation). Figure 7 In the example shown, the off-period Toff11 of thermistor Rd1 ends at timing t4. This reduces the temperature difference between thermistors Rd1 and Rd2 at the start of the dummy heating operation, further reducing the difference in thermal history between thermistors Rd1 and Rd2. This eliminates the need to heat thermistors Rd1 and Rd2 simultaneously during the dummy heating operation.

[0086] Figure 8 2 is a flowchart for explaining the operation of the sensor unit 20. Figure 9 It is a timing chart for explaining the operation of the sensor unit 20 .

[0087] When the gas concentration measurement operation is performed 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). Figure 9 The timing t30 is immediately before the timing t6 at which thermistors Rd3 and Rd4 start to be heated by heaters MH3 and MH4.

[0088] Next, the control circuit 46 included in the signal processing circuit 40 outputs the heater instruction value calculated based on the ambient temperature to the DA converter 45, thereby starting the heating of the thermistors Rd3 and Rd4 (step 202). The heater instruction value is converted into heater voltages Vmh3 and Vmh4 by the DA converter 45 and 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 the thermistor Rd4 is heated to 150°C). The heating of the thermistors Rd3 and Rd4 begins at Figure 9 It is performed at the timing t6 shown.

[0089] The temperatures of thermistors Rd3 and Rd4 remain unstable from the start of heating at time t6 until a predetermined time has passed. Therefore, a predetermined waiting time is required from the start of heating to the sampling of detection signal Vgas2. Then, at time t40 after the predetermined waiting time has passed, the signal processing circuit 40 samples detection signal Vgas2 (step 203). The signal processing circuit 40 then calculates output signal Vout from detection signal Vgas2 and outputs output signal Vout to the outside.

[0090] Next, the control circuit 46 stops heating the thermistors Rd3 and Rd4 by resetting the heater instruction value (step 204). Figure 9 The above is performed at timing t7 shown. The gas concentration measurement operation using sensor unit 20 is completed. During the gas concentration measurement operation using sensor unit 20, the period from timing t6, when heating of thermistors Rd3 and Rd4 is started, to timing t7, when heating of thermistors Rd3 and Rd4 is stopped, is defined as on-period Ton3. On-period Ton3 may be the same as on-period Ton1 in sensor unit 10.

[0091] After the predetermined off-period Toff3 has elapsed from 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 outputs a heater instruction value calculated based on the ambient temperature, thereby starting to heat the thermistors Rd3 and Rd4 (step 202). The heating of the thermistors Rd3 and Rd4 begins at Figure 9 The OFF period Toff3 is defined as the interval from the timing t7 at which heating of thermistors Rd3 and Rd4 ends to the timing t8 at which heating of thermistors Rd3 and Rd4 resumes. The OFF period Toff3 may be shorter than the OFF period Toff2 in the sensor unit 10. The OFF period Toff3 may be the same as the OFF period Toff1 in the sensor unit 10, or may be shorter than the OFF period Toff1 in the sensor unit 10.

[0092] By repeatedly executing such an operation at a predetermined period, the concentration of the detection target gas contained in the environment can be periodically detected.

[0093] In this way, in the sensor unit 10, the gas concentration measurement operation and the dummy heating operation are performed alternately. In contrast, in the sensor unit 20, there is no dummy heating operation (heating the thermistor Rd3 in a temperature range near 150°C (second temperature range) for a prescribed period by the heater MH3, and heating the thermistor Rd4 in a temperature range near 300°C (first temperature range) for a prescribed period by the heater MH4), but the gas concentration measurement operation is continuously performed with the off period Toff3 interposed therebetween.

[0094] Figure 10 Is used to illustrate Figure 2 The timing chart of the operation of the sensor units 10 and 20 in the operation period T1 is shown.

[0095] like Figure 10 As shown, during the operation period T1, the gas concentration measurement operation in the sensor section 20 is performed at a higher frequency than the gas concentration measurement operation in the sensor section 10. The execution period C1 of the gas concentration measurement operation in the sensor section 10 is defined by the period from timing t1 to timing t5. The execution period C2 of the gas concentration measurement operation in the sensor section 20 is defined by the period from timing t6 to timing t8. The execution period C2 is shorter than the execution period C1, and thus, during the operation period T1, the number of executions of the gas concentration measurement operation in the sensor section 20 per unit time is greater than the number of executions of the gas concentration measurement operation in the sensor section 10 per unit time. The execution period C2 can be shortened because, as described above, no dummy heating operation is performed during the period when the gas concentration measurement operation is continuously performed on the sensor section 20. In addition, during Figure 2 During the operation period T3 shown in (b), since the operation of the sensor unit 10 is stopped, the gas concentration measurement operation in the sensor unit 20 is of course performed at a higher frequency than the gas concentration measurement operation in the sensor unit 10, and the number of executions of the gas concentration measurement operation in the sensor unit 20 per unit time is greater than the number of executions of the gas concentration measurement operation in the sensor unit 10 per unit time.

[0096] Thus, during operation period T1, stable gas concentration measurement with little temporal variation can be performed using sensor unit 10, and by using high-frequency gas concentration measurement using sensor unit 20, even when the CO2 gas concentration fluctuates significantly within a short period of time, the output signal Vout can track the concentration change. Furthermore, during operation period T2, stable gas concentration measurement with little temporal variation can be performed using sensor unit 10, and during operation period T3, high-frequency gas concentration measurement using sensor unit 20 can be performed, even when the CO2 gas concentration fluctuates significantly within a short period of time, the output signal Vout can track the concentration change.

[0097] The gas concentration measurement operation using the sensor unit 10 and the gas concentration measurement operation using the sensor unit 20 may be performed asynchronously or at different times. For example, if the timing of the gas concentration measurement operation of the sensor unit 10 and the gas concentration measurement operation of the sensor unit 20 is different so that Figure 4 The timing t20 and Figure 9 If the timings t40 shown do not overlap, the amplified signal Vamp1 and the amplified signal Vamp2 are not input to the AD converter 44 at the same time.

[0098] If used Figure 2 As described above, the gas concentration measurement operation using the sensor unit 20 does not need to be performed all the time, and may be performed intermittently. Figure 11 As shown, 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 the gas concentration measurement operation using the sensor unit 20 may be stopped in response to a stop signal E supplied from the external device. Figure 1 As shown, the start signal S and the stop signal E are supplied to the control circuit 46. Alternatively, a timer or the like may be provided in the signal processing circuit 40 to automatically start the gas concentration measurement operation using the sensor unit 20 at a predetermined timing and automatically end the gas concentration measurement operation using the sensor unit 20 after a predetermined time has elapsed.

[0099] As described above, for the sensor unit 20, the dummy heating operation is not performed during the period in which the gas concentration measurement operation is continuously performed. Therefore, a thermal history difference is generated between the thermistor Rd3 and the thermistor Rd4, and thus the sensor unit 20 changes over time. For example, if the cumulative execution time of the gas concentration measurement operation of the sensor unit 20 in 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 temporal change of the sensor unit 20 can be suppressed. In the operation period T2, although the gas concentration measurement operation using the sensor unit 20 is stopped, 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. In order to suppress the temporal change of the sensor unit 20, the operation period T1 or T3 can also be made shorter than Figure 2 The operation period T2 shown is short.

[0100] Alternatively, during operation period T1, the value of output signal Vout calculated based on detection signal Vgas1 is compared with the value of output signal Vout calculated based on detection signal Vgas2. If a non-negligible difference occurs between the two, it is determined that drift in output signal Vout due to residual heat has occurred in sensor unit 20 due to a short execution cycle C2, and operation period T2 may be extended to negligible the difference. Alternatively, if it is determined that the difference in thermal history between thermistors Rd3 and Rd4 has increased, indicating a significant change in sensor unit 20 over time, gas concentration measurement using sensor unit 20 may be prohibited. Furthermore, based on the difference between the values ​​of output signal Vout calculated based on detection signal Vgas1 and the values ​​of output signal Vout calculated based on detection signal Vgas2, a correction may be applied to the calculation formula used to calculate output signal Vout based on detection signal Vgas2 to remove drift and time-varying components.

[0101] Figure 12 This is a flowchart for explaining the operation according to the modified example of the sensor unit 20 .

[0102] exist Figure 12 In the illustrated variation, the sensor unit 20 performs not only 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 the thermistors Rd3 and Rd4 (step 302). The heater instruction value is converted into heater voltages Vmh3 and Vmh4 by the DA converter 45 and applied to heaters MH3 and MH4, respectively. In step 302, thermistor Rd3 is heated to approximately 300°C and thermistor Rd4 is heated to approximately 150°C (if the CO2 gas concentration in the measurement atmosphere is, for example, zero, thermistor Rd3 is heated to 300°C and thermistor Rd4 is heated to 150°C).

[0103] When the predetermined standby time has elapsed, the signal processing circuit 40 samples the detection signal Vgas2 (step 303). The signal processing circuit 40 then 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 thermistors Rd3 and Rd4 by resetting the heater instruction value (step 304). During the gas concentration measurement operation using the sensor unit 20, the period from the start of heating thermistors Rd3 and Rd4 in step 302 to the stop of heating thermistors Rd3 and Rd4 in step 304 is defined as the on-period Ton3.

[0104] Next, the control circuit 46 determines whether the gas concentration measurement operation using the sensor unit 20 has been stopped (step 305). Alternatively, the determination of whether the gas concentration measurement operation using the sensor unit 20 has been stopped can be based on whether a stop signal E has been input. If it is determined that the gas concentration measurement operation using the sensor unit 20 has not been stopped, the process returns to step 301 to calculate the ambient temperature, and then reheating of the thermistors Rd3 and Rd4 is initiated. During the gas concentration measurement operation using the sensor unit 20, the period from the cessation of heating of thermistors Rd3 and Rd4 to the resumption of heating is defined as the off-period Toff3. On the other hand, if it is determined that the gas concentration measurement operation using the sensor unit 20 has been stopped, the control circuit 46 stores the total heating time (the total time of the on-period Ton3) of the thermistors Rd3 and Rd4 during the gas concentration measurement operation using the sensor unit 20 (step 306), stops the gas concentration measurement operation using the sensor unit 20, and then initiates a dummy heating operation of the sensor unit 20 (step 307).

[0105] In the dummy heating operation of the sensor unit 20, dummy heating of thermistors Rd3 and Rd4 is first started (step 308). During the dummy heating of thermistors Rd3 and Rd4, thermistor Rd3 is heated to approximately 150°C and thermistor Rd4 is heated to approximately 300°C. Next, the control circuit 46 stops heating thermistors Rd3 and Rd4 (step 309). In the dummy heating operation of the sensor unit 20, the period from the start of heating of thermistors Rd3 and Rd4 in step 308 to the stop of heating of thermistors Rd3 and Rd4 in step 309 is defined as the on-period Ton4. The length of the on-period Ton4 may be the same as the length of the on-period Ton3.

[0106] Next, the control circuit 46 determines whether the total heating time of thermistors Rd3 and Rd4 during the dummy heating operation of the sensor unit 20 (the total time of the on-period Ton4) is less than the total heating time of thermistors Rd3 and Rd4 during the gas concentration measurement operation using the sensor unit 20 (the total time of the on-period Ton3) (step 310). If the total time of the on-period Ton4 is less than the total time of the on-period 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 can also be determined based on whether the start signal S is input. If it is determined that the dummy heating operation of the sensor unit 20 is not to be stopped, the control circuit 46 returns to step 308 and starts dummy heating of thermistors Rd3 and Rd4 again. In the dummy heating operation of the sensor unit 20 , a period from when the dummy heating of the thermistors Rd3 and Rd4 is stopped to when the dummy heating is started is defined as an off period Toff4 .

[0107] On the other hand, when it is determined in step 310 that the total time of the on-period Ton4 is greater than the total time of the on-period Ton3, that is, when it is determined that the total heating time during the dummy heating operation of the sensor part 20 is greater than the total heating time during the gas concentration measurement operation using the sensor part 20, or when it is determined in step 311 that the dummy heating operation of the sensor part 20 is stopped, the control circuit 46 stores the total heating time of the thermistors Rd3 and Rd4 during the dummy heating operation of the sensor part 20 (the total time of the on-period Ton4) (step 312), and stops the dummy heating operation of the sensor part 20 (step 313).

[0108] Figure 13 This is a timing chart for explaining an example of the operation according to the modified example of the sensor unit 20 .

[0109] exist Figure 13 In the example shown, a dummy heating operation of the sensor unit 20 is performed in the period 402 from the time when the gas concentration measurement operation using the sensor unit 20 is performed in the period 401 to the time when the gas concentration measurement operation using the sensor unit 20 is performed again in the period 403. Both the periods 401 and 403 are started in response to the start signal S and are ended in response to the stop signal E. On the other hand, the period 402 may be started at a time when an arbitrary standby time has passed after the end of the period 401. In addition, the length of the on-period Ton3 does not need to be the same as the length of the on-period Ton4, and may be Figure 14In the example shown, the length of the on-period Ton4 is set to be 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-period Ton4) reaches the total heating time during the gas concentration measurement operation using the sensor unit 20 (the total time of the on-period Ton3).

[0110] The above describes the embodiments of the technology involved in the present disclosure, but the technology involved in the present disclosure is not limited to the above embodiments, and various changes can be made without departing from the scope of the present disclosure, which are obviously also included in the scope of the technology involved in the present disclosure.

[0111] For example, in the above embodiment, thermistors, which are resistors, are used as the temperature sensing elements of the sensor units 10 and 20. However, the present invention is not limited thereto. For example, platinum (Pt) or tungsten (W), which are resistors, may be used as the temperature sensing elements.

[0112] The technology involved in this disclosure includes, but is not limited to, the following structural examples.

[0113] According to one aspect of the present disclosure, a gas sensor comprises: a first sensor unit comprising: first and second temperature-sensing elements connected in series, a first heater for heating the first temperature-sensing element, and a second heater for heating the second temperature-sensing element, and outputting a first detection signal from a connection point between the first temperature-sensing element and the second temperature-sensing element; and a second sensor unit comprising: third and fourth temperature-sensing elements connected in series, a third heater for heating the third temperature-sensing element, and a fourth heater for heating the fourth temperature-sensing element, and outputting a second detection signal from a connection point between the third temperature-sensing element and the fourth temperature-sensing element; and a signal processing circuit which controls the first and second sensor units and calculates the concentration of the detection target gas based on the first and second detection signals, wherein the signal processing circuit alternately repeats a first gas concentration measurement operation and a dummy heating operation when controlling the first sensor unit, wherein the first gas concentration measurement operation The first temperature sensing element is heated to a first temperature range by a first heater, and the second temperature sensing element is heated to a second temperature range by a second heater. A dummy heating operation is performed to heat the first temperature sensing element to the second temperature range by the first heater, and the second temperature sensing element to the first temperature range by the second heater. When controlling the second sensor unit, the signal processing circuit repeatedly performs a second gas concentration measurement operation without performing a heating operation to heat the third temperature sensing element to the second temperature range by a third heater and the fourth temperature sensing element to the first temperature range by a fourth heater. The second gas concentration measurement operation heats the third temperature sensing element to the first temperature range by the third heater and the fourth temperature sensing element to the second temperature range by the fourth heater. During the first operation, the second gas concentration measurement operation is performed at a higher frequency than the first gas concentration measurement operation. Thus, stable gas concentration measurement with little time variation can be performed using the first sensor unit, and high-frequency gas concentration measurement can be performed using the second sensor unit.

[0114] In the above-described gas sensor, the signal processing circuit may execute the first gas concentration measurement operation and the dummy heating operation and stop the second gas concentration measurement operation during the second operation period. This can suppress temporal changes in the second sensor unit.

[0115] In the above-described gas sensor, the signal processing circuit may periodically perform 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 detection target gas.

[0116] In the gas sensor, during the first and second operation periods, the cumulative execution time of the second gas concentration measurement operation may be shorter than the cumulative execution time of the first gas concentration measurement operation. This can suppress temporal changes in the second sensor unit.

[0117] In the aforementioned gas sensor, the signal processing circuit may perform a second dummy heating operation during the second operation period, wherein the second dummy heating operation heats the third temperature sensing element to the second temperature range using the third heater and heats the fourth temperature sensing element to the first temperature range using the fourth heater. This can suppress temporal changes in the second sensor portion caused by a difference in thermal history between the third and fourth temperature sensing elements.

[0118] In the gas sensor, the temperature of the first temperature region may be higher than the temperature of the second temperature region, and the interval between the operation of heating the first temperature sensing element to the first temperature region 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 region 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 region 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 region by the second heater in the dummy heating operation. Thus, the difference in thermal history between the first temperature sensing element and the second temperature sensing element can be reduced.

[0119] In the gas sensor described above, the interval between the dummy heating operation and the first gas concentration measurement operation may be at least 10 times the execution time of the dummy heating operation. This can reduce measurement errors of the first sensor unit caused by residual heat.

Claims

1. A gas sensor, wherein: have: The first sensor unit includes: first and second temperature sensing elements connected in series; a first heater for heating the first temperature sensing element; and a second heater for heating the second temperature sensing element, and outputs a first detection signal from a connection point between the first temperature sensing element and the second temperature sensing element; a second sensor unit comprising: third and fourth temperature sensing elements connected in series; a third heater for heating the third temperature sensing element; and a fourth heater for heating the fourth temperature sensing element, wherein a second detection signal is output from a connection point between the third temperature sensing element and the fourth temperature sensing element; and a signal processing circuit that controls the first and second sensor units and calculates the concentration of the detection target gas based on the first and second detection signals; The signal processing circuit, when controlling the first sensor unit, alternately and repeatedly performs a first gas concentration measurement operation and a dummy heating operation, The first gas concentration measurement operation heats the first temperature sensing element to a first temperature range by the first heater, and heats the second temperature sensing element to a second temperature range by the second heater; The dummy heating operation heats the first temperature sensing element to the second temperature range through the first heater, and heats the second temperature sensing element to the first temperature range through the second heater. The signal processing circuit repeatedly performs the second gas concentration measurement operation when controlling the second sensor unit without performing the heating operation of heating the third temperature sensing element to the second temperature range by the third heater and heating the fourth temperature sensing element to the first temperature range by the fourth heater, The second gas concentration measurement operation heats the third temperature sensing element to the first temperature range through the third heater, and heats the fourth temperature sensing element to the second temperature range through the fourth heater. During the first operation, the second gas concentration measurement operation is performed at a frequency higher than the first gas concentration measurement operation.

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

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

4. The gas sensor according to claim 2, wherein The accumulated execution time of the second gas concentration measurement operation in the first and second operation periods is shorter than the accumulated execution time of the first gas concentration measurement operation.

5. The gas sensor according to claim 2, wherein the signal processing circuit performing a second dummy heating operation during the second operation, The second dummy heating operation heats the third temperature sensing element to the second temperature range through the third heater, and heats the fourth temperature sensing element to the first temperature range through the fourth heater.

6. The gas sensor according to claim 1, wherein The temperature of the first temperature region is higher than the temperature of the second temperature region. An interval from the operation of heating the first temperature sensing element to the first temperature region by the first heater in the first gas concentration measurement operation to the operation of heating the first temperature sensing element to the second temperature region by the first heater in the dummy heating operation is longer than an interval from the operation of heating the second temperature sensing element to the second temperature region by the second heater in the first gas concentration measurement operation to the operation of heating the second temperature sensing element to the first temperature region by the second heater in the dummy heating operation.

7. The gas sensor according to any one of claims 1 to 6, wherein An interval from the dummy heating operation to the first gas concentration measuring operation is more than 10 times the execution time of the dummy heating operation.

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