Gas sensor

Abstract

Provided is a gas sensor capable of following a concentration change even when the concentration of a detection target gas greatly changes in a short time while suppressing power consumption. A gas sensor (100) includes a sensor unit (11, 21) and a signal processing circuit (40). The signal processing circuit (40) repeatedly executes a first gas concentration measurement operation of acquiring a detection signal (Vgas12) by controlling the sensor unit (11), in response to the gas concentration becoming equal to or greater than a first threshold value, starts and repeatedly executes a second gas concentration measurement operation of acquiring a detection signal (Vgas34) by controlling the sensor unit (21), and in response to the gas concentration being less than a second threshold value, stops the second gas concentration measurement operation. The second gas concentration measurement operation is executed more frequently than the first gas concentration measurement operation.

Description

Technical Field

[0001] This disclosure relates to a gas sensor, and more particularly, to a gas sensor capable of measuring the concentration of a target gas whose concentration may change dramatically over a short period of time. 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 reduces the thermal history difference between the detection thermistor and the reference thermistor by setting a dummy heating period after the measurement operation, ensuring that the heating temperature of the detection thermistor during the measurement period matches the heating temperature of the reference thermistor during the dummy heating period, and also ensuring that the heating temperature of the reference thermistor during the measurement period matches the heating temperature of the detection thermistor during the dummy heating period.

[0003] Existing technical documents

[0004] Patent documents

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

[0006] However, a problem exists: if the gas concentration measurement is performed infrequently, the measurement results cannot keep up with the concentration changes when the concentration of the gas being measured fluctuates significantly within a short period. To enable the measurement results to follow concentration changes, the frequency of gas concentration measurement can be increased, but this would increase power consumption.

[0007] This disclosure describes a technique that enables measurement results to follow concentration changes even when the concentration of the target gas changes significantly over a short period of time, while suppressing power consumption.

[0008] A gas sensor according to one aspect of this disclosure includes: a first sensor unit that outputs a first detection signal corresponding to the concentration of a target gas; a second sensor unit that outputs a second detection signal corresponding to the concentration of the target gas; and a signal processing circuit that controls the first sensor unit and the second sensor unit, and calculates the concentration of the target gas based on the first and second detection signals. The signal processing circuit repeatedly executes a first gas concentration measurement operation by controlling the first sensor unit to acquire the first detection signal. In response to a gas concentration calculated from the first detection signal becoming a first threshold, a second gas concentration measurement operation by controlling the second sensor unit to acquire the second detection signal is started and repeatedly executed. In response to at least one of the gas concentration calculated from the first detection signal and at least the gas concentration calculated from the second detection signal being less than a second threshold, the second gas concentration measurement operation is stopped. The number of times the second gas concentration measurement operation is executed per unit time during the period from the start to the stop of the second gas concentration measurement operation is greater than the number of times the first gas concentration measurement operation is executed per unit time during the period of repeatedly executing the first gas concentration measurement operation.

[0009] According to this disclosure, a technique is provided that can suppress power consumption while enabling measurement results to follow concentration changes even when the concentration of the target gas changes significantly in a short period of time. Attached Figure Description

[0010] Figure 1 This is a circuit diagram illustrating the configuration of a gas sensor 100 according to a first embodiment of the technology disclosed herein.

[0011] Figure 2 (a) and (b) are schematic diagrams illustrating an example of the gas concentration measurement operation performed by the sensor units 11 and 21.

[0012] Figure 3 It is used for explanation Figure 2 The flowchart shown in (a) is an example of a gas concentration measurement operation.

[0013] Figure 4 (a) and (b) are illustrations showing an example of the change in CO2 gas concentration in an atmosphere with controlled CO2 gas concentration. Figure 4 (a) shows an example where the first threshold is the same as the second threshold. Figure 4 (b) shows an example where the first threshold is different from the second threshold.

[0014] Figure 5 It is used for explanation Figure 2 The flowchart shown in (b) is an example of a gas concentration measurement operation.

[0015] Figure 6This is a flowchart illustrating the operation of the sensor unit 11.

[0016] Figure 7 This is a timing diagram used to explain the operation of the sensor unit 11.

[0017] Figure 8 This is a graph showing the relationship between the ratio of Toff2 during the off period to Ton2 during the on period (Toff2 / Ton2) and the amount of drift per unit time in the CO2 gas concentration measurement.

[0018] Figure 9 This is a diagram showing the change in the output signal Vout in an atmosphere with controlled CO2 gas concentration.

[0019] Figure 10 This is a timing diagram used to illustrate the operation of a modified example of the sensor unit 11.

[0020] Figure 11 This is a flowchart illustrating the operation of the sensor unit 21.

[0021] Figure 12 This is a timing diagram used to explain the operation of the sensor unit 21.

[0022] Figure 13 It is used to explain in Figure 2 (a) shows the timing diagram of the operation of sensor units 11 and 21 during operation periods T1 and T2.

[0023] Figure 14 It is used for explanation Figure 2 The flowchart of a variation of the gas concentration measurement operation shown in (a) is as follows.

[0024] Figure 15 It is used to explain in Figure 2 (b) shows the timing diagram of the operation of sensor units 11 and 21 during operation periods T1 and T3.

[0025] Figure 16 It is used for explanation Figure 2 The flowchart of the first variation of the gas concentration measurement operation shown in (b) is as follows.

[0026] Figure 17 It is used for explanation Figure 2 The flowchart of the second variation of the gas concentration measurement operation shown in (b) is as follows.

[0027] Figure 18 It is used to supplement Figure 17 The flowchart shown is a timing diagram.

[0028] Figure 19 It is used for explanation Figure 2The flowchart of the third variation of the gas concentration measurement operation shown in (b) is as follows.

[0029] Figure 20 It is used to supplement Figure 19 The flowchart shown is a timing diagram.

[0030] Figure 21 This is a timing diagram used to illustrate the operation of the first modified example of the sensor unit 21.

[0031] Figure 22 This is a flowchart illustrating the operation of a second modified example of the sensor unit 21.

[0032] Figure 23 This is a timing diagram illustrating an example of the operation of a second modified version of the sensor unit 21.

[0033] Figure 24 This is a timing diagram illustrating another example of the operation of the second modified version of the sensor unit 21.

[0034] Figure 25 This is a circuit diagram illustrating the configuration of a gas sensor 200 according to a second embodiment of the technology disclosed herein.

[0035] Explanation of symbols

[0036] Sensors 11, 12, 21, 22

[0037] 30 Temperature Sensor

[0038] 40 Signal Processing Circuit

[0039] 41 Differential Amplifier

[0040] 41, 42, 47, 48 Differential amplifiers

[0041] 43 Buffer

[0042] 44 AD converter

[0043] 45 DA Converter

[0044] 46 Control Circuit

[0045] Steps 51-58, 101-106, 201-204, 301-313, 511-515, 521-524, 561-563, 571, 572

[0046] 100, 200 gas sensors

[0047] During periods 401 and 402

[0048] C1, C2 execution cycle

[0049] MH1~MH4 heaters

[0050] Connection points N1~N5, N12, N34

[0051] Fixed resistors R1 to R5

[0052] Rd1~Rd5 Thermistors

[0053] During the T1~T3 action period

[0054] Vamp1~Vamp3 Amplified Signal

[0055] Detection signals of Vgas1~Vgas4, Vgas12, and Vgas34

[0056] Heater voltages Vmh1~Vmh4

[0057] Vout output signal

[0058] Vref reference signal

[0059] Vtemp temperature detection signal. Detailed Implementation

[0060] Hereinafter, with reference to the accompanying drawings, embodiments of the technology disclosed herein will be described in detail.

[0061] Figure 1 This is a circuit diagram illustrating the configuration of a gas sensor 100 according to a first embodiment of the technology disclosed herein.

[0062] like Figure 1 As shown, the gas sensor 100 of this embodiment includes two sensor units 11 and 21 for detecting the concentration of the target gas, a temperature sensor 30, and a signal processing circuit 40. Although not particularly limited, the gas sensor 100 of this embodiment is a heat conduction type gas sensor for detecting the concentration of CO2 gas in a measurement atmosphere.

[0063] The sensor unit 11 includes thermistors Rd2 and Rd1 connected in series between the power supply Vcc and ground GND, and heaters MH1 and MH2 for heating thermistors Rd1 and Rd2 respectively. The detection signal Vgas12 of the sensor unit 11 appears at the connection point N12 of thermistors Rd1 and Rd2. Thermistor Rd2 is a temperature sensing element for detection, and thermistor Rd1 is a temperature sensing element for reference. Thermistors Rd1 and Rd2 are resistive elements whose resistance changes with temperature. Examples of materials for thermistors Rd1, Rd2, and the following thermistors Rd3, Rd4, and Rd5 include vanadium oxide, amorphous silicon, polycrystalline silicon, manganese-containing spinel-type crystal structure oxides, titanium oxide, or yttrium-barium-copper oxide. For example, thermistors Rd1, Rd2, Rd3, Rd4, and Rd5 are NTC thermistors with a negative temperature coefficient of resistance. During gas concentration measurement, thermistor Rd1 is heated to approximately 300°C (an example of a first temperature region) via heater MH1, and thermistor Rd2 is heated to approximately 150°C (an example of a second temperature region) via heater MH2. The first temperature region is a defined temperature range, for example, encompassing temperatures above 250°C and below 450°C, such as a temperature region around 300°C. The second temperature region is a defined temperature range, for example, encompassing temperatures above 100°C and below 230°C, such as a temperature region around 150°C. The term "temperature region" in this specification has a temperature width of, for example, within 1°C. For example, a temperature region around 150°C could be a region between 149.5°C and 150.5°C. Furthermore, a temperature region around 300°C could be a region between 299.5°C and 300.5°C. Thermistor Rd1 is designed to achieve a specified resistance value when heated to, for example, 300°C, while thermistor Rd2 is designed to achieve a specified resistance value when heated to, for example, 150°C. The first temperature region (around 300°C in this example) is different from the second temperature region (around 150°C in this example), in which the first temperature region is hotter than the second temperature region.

[0064] When the thermistor Rd2, the temperature-sensing element used for detection, is heated to approximately 150°C, and CO2 gas is present in the measuring atmosphere, the heat dissipation characteristics of the thermistor Rd2 change accordingly with its concentration. This change manifests as a change in the temperature of the thermistor Rd2, i.e., a change in its resistance value. Specifically, since CO2 gas has lower heat dissipation than air, the higher the CO2 gas concentration, the higher the temperature of the thermistor Rd2. Therefore, if heating is performed with, for example, zero CO2 gas concentration in the measuring atmosphere, to bring the temperature of the thermistor Rd2 to 150°C, then when CO2 gas is present in the measuring atmosphere, the temperature of the thermistor Rd2 will correspondingly exceed 150°C. As a result, the higher the CO2 gas concentration in the measuring atmosphere, the lower the resistance value of the thermistor Rd2.

[0065] On the other hand, when the reference temperature sensing element, i.e., the thermistor Rd1, is heated to approximately 300°C, even if CO2 gas is present in the measuring atmosphere, the heat dissipation characteristics of the thermistor Rd1 hardly change accordingly with its concentration, and the temperature of the thermistor Rd1 also remains almost unchanged. Therefore, the change in resistance value of the thermistor Rd1 heated to approximately 300°C due to the CO2 gas concentration is significantly less than the change in resistance value of the thermistor Rd2 heated to approximately 150°C due to the CO2 gas concentration. The change in resistance value of the thermistor Rd1 heated to approximately 300°C due to the CO2 gas concentration can be virtually negligible. As a result, when thermistor Rd2 is heated to around 150°C and thermistor Rd1 is heated to around 300°C (assuming the CO2 gas concentration in the measuring atmosphere is, for example, zero, thermistor Rd1 is heated to 300°C and thermistor Rd2 to 150°C), a detection signal Vgas12 corresponding to the CO2 gas concentration in the measuring atmosphere appears at the connection point N12 of thermistors Rd1 and Rd2. On the other hand, even if the measuring atmosphere contains other gases whose heat dissipation characteristics when thermistor Rd2 is heated to around 150°C are not significantly different from those when thermistor Rd1 is heated to around 300°C, the concentration of these gases has almost no effect on the detection signal Vgas12. Therefore, the sensor unit 11 can selectively detect the CO2 gas concentration.

[0066] Sensor unit 21 has the same circuit structure as sensor unit 11. That is, sensor unit 21 includes thermistors Rd4 and Rd3 connected in series between the power supply Vcc and ground GND, and heaters MH3 and MH4 that heat thermistors Rd3 and Rd4 respectively. The detection signal Vgas34 of sensor unit 21 appears at the connection point N34 of thermistors Rd3 and Rd4. Thermistor Rd4 is a temperature-sensing element for detection and may have the same structure as thermistor Rd2 included in sensor unit 11. Thermistor Rd3 is a temperature-sensing element for reference and may have the same structure as thermistor Rd1 included in sensor unit 11. Thermistors Rd3 and Rd4 are resistive elements whose resistance changes with temperature. During gas concentration measurement, thermistor Rd3 is heated to approximately 300°C (an example of the first temperature range) by heater MH3, and thermistor Rd4 is heated to approximately 150°C (an example of the second temperature range) by heater MH4. Thermistor Rd3, like thermistor Rd1 included in sensor unit 11, is designed to achieve a predetermined resistance value when heated to, for example, 300°C, while thermistor Rd4, like thermistor Rd2 included in sensor unit 11, is designed to achieve a predetermined resistance value when heated to, for example, 150°C.

[0067] Temperature sensor 30 includes a thermistor Rd5 and a fixed resistor R5 connected in series between the power supply Vcc and ground GND. The temperature detection signal Vtemp of temperature sensor 30 appears at the connection point N5 of the thermistor Rd5 and the fixed resistor R5. Temperature sensor 30 detects the ambient temperature. Ambient temperature refers to the temperature of the measured atmosphere. Temperature sensor 30 can be designed to be unaffected or less susceptible to heating by, for example, heaters MH1, MH2, MH3, and MH4.

[0068] 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.

[0069] Differential amplifier 41 compares the detection signal Vgas12 and the reference signal Vref to generate an amplified signal Vamp1, which amplifies the level difference (=Vgas12-Vref) between the detection signal Vgas12 and the reference signal Vref. Differential amplifier 42 compares the detection signal Vgas34 and the reference signal Vref to generate an amplified signal Vamp2, which amplifies the level difference (=Vgas34-Vref) between the detection signal Vgas34 and the reference signal Vref. Buffer 43 buffers the temperature detection signal Vtemp to generate an amplified signal Vamp3. Amplified signals Vamp1 to Vamp3 are input to AD converter 44. AD converter 44 performs AD conversion on amplified signals Vamp1 to Vamp3 to generate digital values ​​and provides them to control circuit 46.

[0070] The control circuit 46 calculates the concentration of the target gas, CO2, based on the amplified signal Vamp1 or Vamp2 converted by the analog-to-digital converter (ADC), and generates an output signal Vout representing the CO2 concentration. The CO2 concentration is calculated using a formula set within the control circuit 46. Furthermore, the control circuit 46 provides digital values ​​of various control parameters to the DA converter 45. The DA converter 45 generates heater voltages Vmh1 to Vmh4 and a reference signal Vref by performing analog-to-analog conversion on the digital values ​​of the various control parameters. The heater voltages Vmh1 to Vmh4 are applied to heaters MH1 to MH4, thereby heating thermistors Rd1 to Rd4. Additionally, the reference signal Vref is provided to differential amplifiers 41 and 42.

[0071] The control circuit 46 corrects the heater voltages Vmh1 to Vmh4 based on the amplified signal Vamp3 after AD conversion. That is, regardless of the ambient temperature, the heater voltages Vmh1 to Vmh4 are corrected so that when the CO2 gas concentration in the measured atmosphere is, for example, zero, the temperatures of thermistors Rd2 and Rd4 are, for example, 150°C, and the temperatures of thermistors Rd1 and Rd3 are, for example, 300°C.

[0072] Next, the operation of the gas sensor 100 in this embodiment will be explained.

[0073] The gas sensor 100 of this embodiment performs both of the following: generating an output signal Vout using the detection signal Vgas12 from the sensor unit 11, that is, generating an output signal Vout using the amplified signal Vamp1; and generating an output signal Vout using the detection signal Vgas34 from the sensor unit 21, that is, generating an output signal Vout using the amplified signal Vamp2. The signal processing circuit 40 controls the sensor unit 11 to suppress changes over time, and controls the sensor unit 21 so that the concentration of the target gas can be accurately detected even when the concentration of the target gas changes significantly in a short period of time.

[0074] Figure 2 (a) and (b) are schematic diagrams illustrating an example of the gas concentration measurement operation performed by the sensor units 11 and 21.

[0075] exist Figure 2 In the example shown in (a), during operation period T1, the gas concentration measurement operation of sensor unit 11 is performed, while the gas concentration measurement operation of sensor unit 21 is stopped; during operation period T2, the gas concentration measurement operation of sensor unit 21 is performed, while the gas concentration measurement operation of sensor unit 11 is stopped. Operation period T1 and operation period T2 are separate periods and do not overlap. Operation period T2 may be shorter than operation period T1.

[0076] exist Figure 2 In the example shown in (b), during operation period T3, the gas concentration measurement operations of sensor unit 11 and sensor unit 21 are performed in parallel; during operation period T1, the gas concentration measurement operation of sensor unit 11 is performed, while the gas concentration measurement operation of sensor unit 21 is stopped. Operation period T1 and operation period T3 are separate periods and do not overlap. Operation period T3 may be shorter than operation period T1.

[0077] Figure 3 It is used for explanation Figure 2 The flowchart shown in (a) is an example of a gas concentration measurement operation.

[0078] exist Figure 3In the example shown, firstly, the gas concentration measurement operation of sensor unit 11 begins (step 51). When a detection signal Vgas12 appears at the connection point N12 of thermistors Rd1 and Rd2 through the gas concentration measurement operation of sensor unit 11, the signal processing circuit 40 acquires the detection signal Vgas12 and generates an output signal Vout based on the detection signal Vgas12 (step 52). Then, the CO2 gas concentration represented by the output signal Vout is calculated, and it is determined whether it is above a first threshold (step 53). The CO2 gas concentration equivalent to the first threshold is arbitrary, for example, 1000 ppm. If the determination result is that the current CO2 gas concentration is less than the first threshold, the process returns to step 52, and the acquisition of the detection signal Vgas12 and the generation of the output signal Vout are performed again. On the other hand, if the determination result is that the current CO2 gas concentration is above the first threshold, the gas concentration measurement operation of sensor unit 11 is stopped (step 54). During these steps 51 to 54, the gas concentration measurement operation of sensor unit 21 is not performed. In other words, the period from steps 51 to 54 is equivalent to Figure 2 During the action period T1 shown in (a).

[0079] After the gas concentration measurement operation of sensor unit 11 stops, the gas concentration measurement operation of sensor unit 21 begins (step 55). When a detection signal Vgas34 appears at the connection point N34 of thermistors Rd3 and Rd4 through the gas concentration measurement operation of sensor unit 21, the signal processing circuit 40 acquires the detection signal Vgas34 and generates an output signal Vout based on the detection signal Vgas34 (step 56). Then, the CO2 gas concentration represented by the output signal Vout is calculated, and it is determined whether it is less than a second threshold (step 57). The CO2 gas concentration equivalent to the second threshold is arbitrary and can be the same as or different from the first threshold (e.g., 1000 ppm). If the determination result is that the current CO2 gas concentration is above the second threshold, the process returns to step 56, and the acquisition of the detection signal Vgas34 and the generation of the output signal Vout are repeated. On the other hand, if the determination result is that the current CO2 gas concentration is less than the second threshold, the gas concentration measurement operation of sensor unit 21 stops (step 58), and the process returns to step 51. During steps 55-58, the gas concentration measurement operation of sensor unit 11 is not performed. In other words, the period from steps 55 to 58 is equivalent to... Figure 2 During the action period T2 shown in (a).

[0080] Figure 4 (a) and (b) are illustrations showing an example of the change in CO2 gas concentration in an atmosphere with controlled CO2 gas concentration. Figure 4(a) shows an example where the first threshold is the same as the second threshold. Figure 4 (b) shows an example where the first threshold is different from the second threshold.

[0081] exist Figure 4 In the examples shown in (a) and (b), the CO2 gas concentration was constant at 400 ppm from the start until 2 minutes later. Then, after 2 minutes, the CO2 gas concentration rose to 5000 ppm, and after 3.5 minutes, the CO2 gas concentration returned to 400 ppm. Figure 4 In the example shown in (a), both the first and second thresholds are 1000 ppm. Figure 4 In the example shown in (b), the first threshold is 1000 ppm and the second threshold is 800 ppm.

[0082] In these cases, during the period from start to 2 minutes, since the CO2 gas concentration is below the first threshold, the gas concentration measurement operation of sensor unit 11 is repeatedly performed. That is, the period from start to 2 minutes is equivalent to... Figure 2 During the operation period T1 shown in (a), after 2 minutes from the start, the gas concentration measurement operation of sensor unit 11 stops because the CO2 gas concentration exceeds the first threshold. Instead, the gas concentration measurement operation of sensor unit 21 is repeated. That is, after 2 minutes from the start, from Figure 2 The operation period T1 shown in (a) transitions to operation period T2. Furthermore, at the point where 3.5 minutes have elapsed since the start, because the CO2 gas concentration is below the second threshold, the gas concentration measurement operation of sensor unit 21 stops, and instead, the gas concentration measurement operation of sensor unit 11 restarts. That is, at the point where 3.5 minutes have elapsed since the start, from... Figure 2 The action period T2 is transitioned to the action period T1 as shown in (a).

[0083] exist Figure 4 In the examples shown in (a) and (b), the CO2 gas concentration temporarily dropped to 2000 ppm after 2 minutes from the start and before 3.5 minutes had passed. However, since the concentration remained above the second threshold, the gas concentration measurement operation of the sensor unit 21 continued. Furthermore, as... Figure 4 As shown in example (b), if hysteresis is provided by setting the second threshold lower than the first threshold, it is difficult to generate an unstable situation in which the switching between action period T1 and action period T2 occurs frequently in a short period of time.

[0084] Figure 5 It is used for explanation Figure 2The flowchart shown in (b) is an example of a gas concentration measurement operation.

[0085] Figure 5 The actions shown are Figure 3 The difference in the actions shown is that step 54 is omitted; step 56 is replaced by steps 561 and 562; step 57 is replaced by steps 571 and 572; and after stopping the gas concentration measurement operation of sensor unit 21 in step 58, the process returns to step 52. Other basic actions are the same as... Figure 3 The actions shown are identical, therefore the same symbols are assigned to the same steps, and repeated descriptions are omitted. Figure 5 During steps 51 to 53, the gas concentration measurement operation of sensor unit 21 is not performed. In other words, the period from steps 51 to 53 is equivalent to... Figure 2 During the operation period T1 shown in (b), when the gas concentration measurement operation of sensor unit 21 starts in step 55, the generation of output signal Vout based on detection signal Vgas12 (step 561) and the generation of output signal Vout based on detection signal Vgas34 (step 562) are executed in parallel. That is, the gas concentration measurement operation of sensor unit 21 continues without interruption. Then, it is determined whether the CO2 gas concentration represented by the output signal Vout generated based on detection signal Vgas12 is less than the second threshold (step 571), and it is determined whether the CO2 gas concentration represented by the output signal Vout generated based on detection signal Vgas34 is less than the second threshold (step 572). If the determination result in step 571 and / or step 572 is that the current CO2 gas concentration is less than the second threshold, the gas concentration measurement operation of sensor unit 21 is stopped (step 58), and the process returns to step 52. Figure 5 The periods shown in steps 55, 561, 562, 571, 572, and 58 are equivalent to Figure 2 During the action period T3 shown in (b).

[0086] exist Figure 5 In the example shown, the determination in steps 571 and 572 can be based on the condition that both the CO2 gas concentrations measured by sensor units 11 and 21 are less than the second threshold, or it can be based on the condition that either the CO2 gas concentration measured by sensor units 11 and 21 is less than the second threshold.

[0087] The operation of sensor units 11 and 21 will be explained in more detail below.

[0088] Figure 6 This is a flowchart illustrating the operation of the sensor unit 11. Furthermore, Figure 7 This is a timing diagram used to explain the operation of the sensor unit 11.

[0089] When performing a gas concentration measurement using sensor unit 11, firstly, the signal processing circuit 40 included in gas sensor 100 samples the temperature detection signal Vtemp and calculates the ambient temperature (step 101). The sampling of the temperature detection signal Vtemp occurs during... Figure 7 The process is carried out at time t10. Time t10 is before time t0, when the thermistors Rd1 and Rd2 will begin to be heated by heaters MH1 and MH2.

[0090] Next, the control circuit 46 included in the signal processing circuit 40 starts heating of thermistors Rd1 and Rd2 by outputting a heater indication value calculated based on the ambient temperature to the DA converter 45 (step 102). The heater indication value is converted by the DA converter 45 into heater voltages Vmh1 and Vmh2, which are applied to heaters MH1 and MH2, respectively. In step 102, thermistor Rd1 is heated to, for example, about 300°C, and thermistor Rd2 is heated to, for example, about 150°C. Heating of thermistors Rd1 and Rd2 begins at... Figure 7 The time t0 shown is taken (when the CO2 gas concentration in the measuring atmosphere is, for example, zero, thermistor Rd1 is heated to, for example, 300°C, and thermistor Rd2 is heated to, for example, 150°C).

[0091] The temperatures of thermistors Rd1 and Rd2 are unstable from the start of heating (t0) until a predetermined time has elapsed. Therefore, a predetermined preparation time is required from the start of heating to the sampling of the detection signal Vgas12. Then, at the predetermined preparation time (t20), the signal processing circuit 40 samples the detection signal Vgas12 (step 103). The signal processing circuit 40 then calculates the output signal Vout from the detection signal Vgas12 and outputs the output signal Vout to the outside.

[0092] Next, control circuit 46 stops the heating of thermistors Rd1 and Rd2 by resetting the heater indication value (step 104). Heating of thermistors Rd1 and Rd2 stops at... Figure 7 The operation is performed at time t1 as shown. Through the above description, the gas concentration measurement operation using sensor unit 11 is completed. In the gas concentration measurement operation using sensor unit 11, the period from the time t0 when heating thermistors Rd1 and Rd2 begins to the time t1 when heating thermistors Rd1 and Rd2 stops is defined as the on-time Ton1.

[0093] After a predetermined off-time Toff1 has elapsed from time t1, control circuit 46 initiates pseudo-heating of thermistors Rd1 and Rd2 (step 105) by outputting a heater indication value calculated based on ambient temperature to DA converter 45. In step 105, thermistor Rd1 is heated to, for example, approximately 150°C (an example of the second temperature region), and thermistor Rd2 is heated to, for example, approximately 300°C (an example of the first temperature region). The heating of thermistors Rd1 and Rd2 begins at... Figure 7 The time t2 shown is taken. Therefore, the turn-off period Toff1 is defined by the time t1 when heating of thermistors Rd1 and Rd2 ends and the time t2 when heating of thermistors Rd1 and Rd2 begins again.

[0094] After the predetermined on-time Ton2 has elapsed from time t2, the control circuit 46 stops the heating of thermistors Rd1 and Rd2 by resetting the heater indication value (step 106). The heating of thermistors Rd1 and Rd2 stops at... Figure 7 The process is carried out at time t3 as shown. Through the above description, the pseudo-heating operation is completed. In the pseudo-heating operation, the period from time t2 when heating thermistors Rd1 and Rd2 begins to time t3 when heating thermistors Rd1 and Rd2 stops is defined as the on-time Ton2.

[0095] After the predetermined shutdown period Toff2 has elapsed from time t3, the control circuit 46 restarts the gas concentration measurement operation. That is, the signal processing circuit 40 samples the temperature detection signal Vtemp, calculates the ambient temperature (step 101), and the control circuit 46 starts heating the thermistors Rd1 and Rd2 by outputting the heater indication value calculated based on the ambient temperature (step 102). Heating of thermistors Rd1 and Rd2 begins at... Figure 7 The time t4 shown is taken. Therefore, the turn-off period Toff2 is defined by the interval from the time t3 when heating of thermistors Rd1 and Rd2 ends to the time t4 when heating of thermistors Rd1 and Rd2 begins again.

[0096] By repeating this action at a predetermined cycle, the concentration of the target gas in the environment can be periodically detected. The execution cycle C1 of the gas concentration measurement action using sensor unit 11 is defined by the period from time t0 to time t4. The execution cycle C1 can be set to suppress the power consumption of gas sensor 100 in an environment where the CO2 gas concentration is relatively stable. For example, the execution cycle C1 is 30 seconds.

[0097] Furthermore, during the on-state period Ton1, when gas concentration measurement is performed, thermistor Rd1 is heated to, for example, approximately 300°C, and thermistor Rd2 is heated to, for example, approximately 150°C. Conversely, during the on-state period Ton2, when spurious heating is performed, thermistor Rd1 is heated to, for example, approximately 150°C, and thermistor Rd2 is heated to, for example, approximately 300°C. Therefore, the difference between the thermal history of thermistor Rd1 and the thermal history of thermistor Rd2 is reduced, and the time-varying changes of the sensor section 11 caused by the thermal history difference are suppressed. To further reduce the thermal history difference, the length of the on-state period Ton1 and the length of the on-state period Ton2 can be set to be the same.

[0098] On the other hand, the lengths of the shutdown period Toff1 and Toff2 do not need to be the same; Toff2 can be longer than Toff1. This is because, since the next gas concentration measurement operation immediately follows Toff2, ensuring the sufficient length of Toff2 reduces measurement errors caused by residual heat. The effect of residual heat manifests, for example, as the drift of the output signal Vout over time.

[0099] Figure 8 This is a graph showing the relationship between the ratio of Toff2 during the off period to Ton2 during the on period (Toff2 / Ton2) and the amount of drift per unit time of the measured CO2 concentration obtained from the output signal Vout in a measurement atmosphere managed as a certain measurement atmosphere with a CO2 gas concentration of 400 ppm.

[0100] like Figure 8 As shown, the larger the ratio of the off-period Toff2 to the on-period Ton2 (Toff2 / Ton2), the smaller the drift per unit time. When the ratio of the off-period Toff2 to the on-period Ton2 is greater than 10, the drift per unit time is almost saturated; when the ratio is greater than 20, the drift per unit time is almost zero. Considering this, to fully suppress the drift of CO2 gas concentration measurement results caused by the influence of residual heat, the off-period Toff2 can be set to more than 10 times or more 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 will become longer. Therefore, the length of the off-period Toff2 can be set according to the purpose.

[0101] Figure 9This is a graph illustrating the variation of the output signal Vout (the measurement result of the CO2 gas concentration obtained from the output signal Vout) in an atmosphere with controlled CO2 gas concentration. The solid line represents the case where the ratio of Toff2 during the off period to Ton2 during the on period (Toff2 / Ton2) is 21.5, and the dashed line represents the case where the ratio of Toff2 during the off period to Ton2 during the on period (Toff2 / Ton2) is 3.63. The CO2 gas concentration is used as a reference value of 400 ppm and varies stepwise to 1000 ppm, 2000 ppm, 3000 ppm, 4000 ppm, and 5000 ppm. The ratio of Toff1 during the off period to Ton1 during the on period (Toff1 / Ton1) is set to a constant ((Toff1 / Ton1) = 5).

[0102] like Figure 9 As shown, it can be seen that when the value of Toff2 / Ton2 is 21.5, the output signal Vout represents the accurate value. In contrast, when the value of Toff2 / Ton2 is 3.63, the drift of the output signal Vout increases over time.

[0103] Figure 10 This is a timing diagram used to illustrate the operation of a modified example of the sensor unit 11.

[0104] like Figure 10 As shown, in the modified operation of sensor unit 11, the off-time Toff11 of the thermistor Rd1 (the interval between the operation of heating the thermistor Rd1 to approximately 300°C (first temperature region) via heater MH1 in the gas concentration measurement operation and the operation of heating the thermistor Rd1 to approximately 150°C (second temperature region) via heater MH1 in the pseudo-heating operation) is set longer than the off-time Toff12 of the thermistor Rd2 (the interval between the operation of heating the thermistor Rd2 to approximately 150°C (second temperature region) via heater MH2 in the gas concentration measurement operation and the operation of heating the thermistor Rd2 to approximately 300°C (first temperature region) via heater MH2 in the pseudo-heating operation). Figure 10 In the example shown, the off-time Toff11 of thermistor Rd1 ends at time t3. Therefore, since the temperature difference between thermistor Rd1 and thermistor Rd2 decreases at the start of the sham heating operation, the thermal history difference between thermistors Rd1 and Rd2 can be further reduced. Thus, during the sham heating operation, it is not necessary to heat thermistors Rd1 and Rd2 simultaneously.

[0105] Figure 11This is a flowchart illustrating the operation of the sensor unit 21. Furthermore, Figure 12 This is a timing diagram used to explain the operation of the sensor unit 21.

[0106] When performing gas concentration measurement using sensor unit 21, firstly, the signal processing circuit 40 included in gas sensor 100 samples the temperature detection signal Vtemp and calculates the ambient temperature (step 201). The sampling of the temperature detection signal Vtemp occurs during... Figure 12 The process takes place at time t30. Time t30 is before time t5, when the thermistors Rd3 and Rd4 will begin to heat through heaters MH3 and MH4.

[0107] Next, the control circuit 46 included in the signal processing circuit 40 starts heating thermistors Rd3 and Rd4 by outputting the heater indication value calculated based on the ambient temperature to the DA converter 45 (step 202). The heater indication value is converted by the DA converter 45 into heater voltages Vmh3 and Vmh4, and applied to heaters MH3 and MH4 respectively. In step 202, thermistor Rd3 is heated to, for example, about 300°C, and thermistor Rd4 is heated to, for example, about 150°C (if the CO2 gas concentration in the measuring atmosphere is, for example, zero, thermistor Rd3 is heated to, for example, 300°C, and thermistor Rd4 is heated to, for example, 150°C). Heating of thermistors Rd3 and Rd4 begins at... Figure 12 The event occurs at time t5 as shown.

[0108] The temperatures of thermistors Rd3 and Rd4 are unstable from the start of heating (t5) until a predetermined time has elapsed. Therefore, a predetermined preparation time is required from the start of heating to the sampling of the detection signal Vgas34. Then, the signal processing circuit 40 samples the detection signal Vgas34 at the predetermined preparation time (t40) (step 203). The signal processing circuit 40 then calculates the output signal Vout from the detection signal Vgas34 and outputs the output signal Vout to the outside.

[0109] Next, control circuit 46 stops the heating of thermistors Rd3 and Rd4 by resetting the heater indication value (step 204). Heating of thermistors Rd3 and Rd4 stops at... Figure 12 The operation is performed at time t6 as shown. Through the above description, the gas concentration measurement operation using sensor unit 21 is completed. In the gas concentration measurement operation using sensor unit 21, the period from the start of heating thermistors Rd3 and Rd4 at time t5 to the stop of heating thermistors Rd3 and Rd4 at time t6 is defined as the on-time Ton3. The on-time Ton3 can be the same as the on-time Ton1 in sensor unit 11.

[0110] After the predetermined shutdown period Toff5 has elapsed from time t6, the control circuit 46 restarts the gas concentration measurement operation. That is, the signal processing circuit 40 samples the temperature detection signal Vtemp, calculates the ambient temperature (step 201), and the control circuit 46 starts heating the thermistors Rd3 and Rd4 by outputting a heater indication value calculated based on the ambient temperature (step 202). Heating of thermistors Rd3 and Rd4 begins at... Figure 12 The turn-off period Toff5 is defined by the interval from the time t6 when heating of thermistors Rd3 and Rd4 ends to the time t9 when heating of thermistors Rd3 and Rd4 begins again. The turn-off period Toff5 can be shorter than the turn-off period Toff2 in the sensor section 11. The turn-off period Toff5 can be the same as or shorter than the turn-off period Toff1 in the sensor section 11.

[0111] By repeating this action at a predetermined period, the concentration of the target gas in the environment can be periodically detected. The execution period C2 of the gas concentration measurement action using sensor unit 21 is defined by the period from time t5 to time t9. The execution period C2 is set to be shorter than the execution period C1, so that even if the CO2 gas concentration changes significantly in a short period of time, the measurement result will follow the concentration change. For example, the execution period C2 is 2 seconds.

[0112] In the above example, in sensor unit 11, the gas concentration measurement operation and the sham heating operation are performed alternately. In contrast, in sensor unit 21, the gas concentration measurement operation is performed continuously during the off period Toff5 without the sham heating operation (a heating operation in which thermistor Rd3 is heated to a temperature range of approximately 150°C (second temperature range) by heater MH3 during a specified period, and thermistor Rd4 is heated to a temperature range of approximately 300°C (first temperature range) by heater MH4 during a specified period).

[0113] Figure 13 It is used to explain in Figure 2 (a) is a timing diagram of an example of the operation of sensor units 11 and 21 during operation periods T1 and T2.

[0114] exist Figure 13In the example shown, the gas concentration measurement operation using sensor unit 21 during operation period T2 is performed at a higher frequency than the gas concentration measurement operation using sensor unit 11 during operation period T1. The execution cycle C1 of the concentration measurement operation using sensor unit 11 during operation period T1 is defined by the period from time t0 to time t4. The execution cycle C2 of the gas concentration measurement operation using sensor unit 21 during operation period T2 is defined by the period from time t5 to time t9. Since execution cycle C2 is shorter than execution cycle C1, the number of times the gas concentration measurement operation using sensor unit 21 is performed per unit time during operation period T2 is greater than the number of times the gas concentration measurement operation using sensor unit 11 is performed per unit time during operation period T1.

[0115] Figure 14 It is used for explanation Figure 2 The flowchart of a variation of the gas concentration measurement operation shown in (a) is as follows.

[0116] Figure 14 The actions of the variant examples shown are similar to Figure 3 The difference in the actions shown is that steps 511-513 are added; and step 56 is replaced by step 514. Other basic actions are the same as... Figure 3 The actions shown are the same, so the same symbols are assigned to the same steps, and repeated descriptions are omitted.

[0117] Step 511 is to Figure 13 The calculated value Vout1(0) shown is stored inside the control circuit 46. The calculated value Vout1(0) is the value of the output signal Vout when it is determined in step 53 that the CO2 gas concentration is above the first threshold. Step 512 is as follows: Figure 13 The diagram shows the step of generating a calculated value Vout2(0) based on the initial detection signal Vgas34 after the gas concentration measurement operation of the start sensor unit 21. The calculated value Vout2(0) is equivalent to the value of the output signal Vout calculated based on the initial detection signal Vgas34 after the gas concentration measurement operation of the start sensor unit 21. The calculated value Vout2(0) is stored inside the control circuit 46 in step 513.

[0118] Step 514 is the step of generating the output signal Vout(n) based on the (n+1)th detection signal Vgas34 (n is an integer greater than or equal to 0). The output signal Vout(n) refers to the output signal Vout generated based on the (n+1)th detection signal Vgas34. In the generation of the output signal Vout(n) in step 514, not only the calculated value Vout2(n) based on the (n+1)th detection signal Vgas34 is used, but also the calculated values ​​Vout1(0) and Vout2(0) stored in the control circuit 46 are used. Specifically, the output signal Vout(n) is calculated by Vout(n) = Vout1(0) + (Vout2(n) - Vout2(0)). That is to say, it is not as... Figure 3 As in step 56, instead of directly generating the output signal Vout based on the current detection signal Vgas34, the output signal Vout(n) is generated by adding the calculated value Vout1(0) generated based on the detection signal Vgas12 when the CO2 gas concentration is determined to be above the first threshold to the difference obtained by subtracting the calculated value Vout2(n) based on the current (n+1)th detection signal Vgas34 from the calculated value Vout2(0) based on the initial detection signal Vgas34. The above formula can be expressed as Vout(n) = Vout2(n) + (Vout1(0) - Vout2(0)). Therefore, in other words, the above content generates the output signal Vout(n) by adding the calculated value Vout2(n) based on the current (n+1)th detection signal Vgas34 to the difference obtained by subtracting the calculated value Vout2(0) from the calculated value Vout1(0). In other words, the calculated value Vout2(n) is corrected by adding the difference obtained by subtracting the calculated value Vout2(0) from the calculated value Vout1(0) to generate the output signal Vout(n).

[0119] Even assuming that sensor unit 11 and sensor unit 21 perform gas concentration measurement operations at the same time, the following situation exists: sensor unit 11 and sensor unit 21 have different characteristics, and the value of the output signal Vout based on the detection signal Vgas12 of sensor unit 11 is different from the value of the output signal Vout based on the detection signal Vgas34 of sensor unit 21. Therefore, when sensor unit 11 and sensor unit 21 have different characteristics, if the output signal Vout is directly generated based on the current detection signal Vgas34, a gas concentration measurement error will occur. However, by generating the output signal Vout(n) in this way, the gas concentration measurement error can be reduced.

[0120] exist Figure 14In the case of the variant shown, in step 57, it is determined whether the CO2 gas concentration represented by the output signal Vout generated based on the calculated value Vout2(n), the calculated value Vout1(0), and the calculated value Vout2(0) is less than the second threshold.

[0121] In the above variation, the calculated value Vout2(0) of the detection signal Vgas34 after the calculated value Vout1(0) is obtained is used to generate the output signal Vout(n). However, instead of using the calculated value Vout2(0) of the detection signal Vgas34 after the calculated value Vout1(0) is obtained, the calculated value Vout2(1) of the second detection signal Vgas34 after the calculated value Vout1(0) is obtained can be used. Accordingly, since the calculated value Vout2(1) obtained in a more stable state is used instead of the calculated value Vout2(0) obtained in the unstable state immediately after the sensor unit 21 is started, a more accurate gas concentration measurement can be performed.

[0122] Alternatively, instead of using the calculated value Vout2(0), the average of multiple calculated values ​​Vout2(0), Vout2(1), Vout2(2), ... of the multiple detection signals Vgas34 obtained after obtaining the calculated value Vout1(0) can be used. This allows for more accurate gas concentration measurements. In this case, the calculated value Vout2(0) may not be included in the calculation of the average value.

[0123] Figure 15 It is used to explain in Figure 2 (b) shows the timing diagram of the operation of sensor units 11 and 21 during operation periods T1 and T3.

[0124] exist Figure 15 In the example shown, the gas concentration measurement operation using sensor unit 21 during operation period T3 is performed at a higher frequency than the gas concentration measurement operation using sensor unit 11 during operation periods T1 and T3. The execution cycle C1 of the concentration measurement operation using sensor unit 11 during operation periods T1 and T3 is defined by the period from time t0 to time t4. The execution cycle C2 of the gas concentration measurement operation using sensor unit 21 during operation period T3 is defined by the period from time t5 to time t9. The execution cycle C2 is shorter than the execution cycle C1; therefore, the number of times the gas concentration measurement operation using sensor unit 21 is performed per unit time during operation period T3 is greater than the number of times the gas concentration measurement operation using sensor unit 11 is performed per unit time during operation periods T1 and T3.

[0125] Figure 16 It is used for explanation Figure 2The flowchart of the first variation of the gas concentration measurement operation shown in (b) is as follows.

[0126] Figure 16 The action of the first variant shown is the same as Figure 5 The difference in the actions shown is that steps 511-513 are added; and step 562 is replaced by step 563. Other basic actions are the same as... Figure 5 The actions shown are the same, so the same symbols are assigned to the same steps, and repeated descriptions are omitted.

[0127] Step 511 is to Figure 15 The calculated value Vout1(0) shown is stored inside the control circuit 46. Step 512 is as follows. Figure 15 The diagram shows the steps for generating the calculated value Vout2(0) based on the initial detection signal Vgas34 after the gas concentration measurement operation of the start sensor unit 21. The calculated value Vout2(0) is stored inside the control circuit 46 in step 513.

[0128] Step 563 is the step of generating the output signal Vout(n) based on the (n+1)th detection signal Vgas34. In the generation of the output signal Vout(n) in step 563, not only is the calculated value Vout2(n) based on the (n+1)th detection signal Vgas34 used, but also the calculated values ​​Vout1(0) and Vout2(0) stored in the control circuit 46. Specifically, the output signal Vout(n) is calculated using Vout(n) = Vout1(0) + (Vout2(n) - Vout2(0)). That is to say, it is not as... Figure 5As in step 562, instead of directly generating the output signal Vout based on the current detection signal Vgas34, the output signal Vout(n) is generated by adding the difference between the calculated value Vout1(0) based on the detection signal Vgas12 when the CO2 gas concentration is determined to be above the first threshold and the calculated value Vout2(n) based on the current (n+1)th detection signal Vgas34 and the calculated value Vout2(0) based on the initial detection signal Vgas34. The above formula can be expressed as Vout(n) = Vout2(n) + (Vout1(0) - Vout2(0)). Therefore, in other words, the above content generates the output signal Vout(n) by adding the difference between the calculated value Vout1(0) and the calculated value Vout2(0) based on the current (n+1)th detection signal Vgas34 and the calculated value Vout2(n). In other words, the calculated value Vout2(n) is corrected by adding the difference obtained by subtracting the calculated value Vout2(0) from the calculated value Vout1(0) to generate the output signal Vout(n). As a result, even if there are differences in characteristics between the sensor unit 11 and the sensor unit 21, the measurement error of the gas concentration can be reduced.

[0129] exist Figure 16 In the case of the first variant shown, in step 572, it is determined whether the CO2 gas concentration represented by the output signal Vout generated based on the calculated value Vout2(n), the calculated value Vout1(0), and the calculated value Vout2(0) is less than the second threshold.

[0130] In the first variation described above, the calculated value Vout2(0) of the detection signal Vgas34 after the calculated value Vout1(0) is obtained is used to generate the output signal Vout(n). However, instead of using the calculated value Vout2(0) of the detection signal Vgas34 after the calculated value Vout1(0) is obtained, the calculated value Vout2(1) of the second detection signal Vgas34 after the calculated value Vout1(0) is obtained can be used. Furthermore, instead of using the calculated value Vout2(0), the average value of multiple calculated values ​​Vout2(0), Vout2(1), Vout2(2)... of the multiple detection signals Vgas34 obtained after the calculated value Vout1(0) can be used. In this case, the calculated value Vout2(0) may not be included in the calculation of the average value.

[0131] Figure 17 It is used for explanation Figure 2 The flowchart of the second variation of the gas concentration measurement operation shown in (b) is as follows. Figure 18 It is used to illustrate the second variation. Figure 2(b) shows the timing diagram of the operation of sensor units 11 and 21 during operation periods T1 and T3.

[0132] Figure 17 The action of the second variation shown is the same as Figure 16 The difference in the action of the first variant shown is that step 511 is replaced by step 515. Accompanying this, the generation time of the calculated value Vout2(0) in step 512 is... Figure 16 The actions shown in the first variation are different. Other basic actions are the same. Figure 16 The actions in the first variant shown are the same, so the same symbols are assigned to the same steps, and repeated descriptions are omitted.

[0133] Step 515 is to Figure 18 The step shown is storing the calculated value Vout1(1) inside the control circuit 46. The calculated value Vout1(1) is equivalent to the value of the output signal Vout calculated in step 53 based on the detection signal Vgas12 obtained from the initial gas concentration measurement operation of the sensor unit 11 after determining that the CO2 gas concentration is above the first threshold. In the second variation, in the generation of the output signal Vout(n) in step 563, not only is the calculated value Vout2(n) of the (n+1)th detection signal Vgas34 after the gas concentration measurement operation of the sensor unit 11 based on the calculated value Vout1(1) used, but also the calculated values ​​Vout1(1) and Vout2(0) stored in the control circuit 46. Figure 17 In the second variant shown, the calculated value Vout2(0) is generated based on the initial detection signal Vgas34 after the gas concentration measurement operation of the sensor unit 11, which calculates the calculated value Vout1(1). Specifically, the output signal Vout(n) is calculated by Vout(n) = Vout1(1) + (Vout2(n) - Vout2(0)).

[0134] That is to say, in the second variation, such as Figure 18As shown in the timing diagram, when it is determined that the CO2 gas concentration is above the first threshold, instead of storing the calculated value Vout1(0) generated by the gas concentration measurement operation of the sensor unit 11 at the time of the determination, the calculated value Vout1(1) generated by the detection signal Vgas12 of the initial gas concentration measurement operation of the sensor unit 11 after determining that the CO2 gas concentration is above the first threshold is stored. The output signal Vout(n) is generated by adding the difference obtained by subtracting the calculated value Vout2(0) of the initial detection signal Vgas34 after the gas concentration measurement operation of the sensor unit 11 based on the calculated value Vout1(1) from the calculated value Vout2(n) based on the detection signal Vgas34. In other words, in the second variation, the calculated value Vout1(1) is used instead of the calculated value Vout1(0) in the first variation to generate the output signal Vout(n). As a result, the time difference between obtaining the calculated value Vout1(1) and obtaining the calculated value Vout2(0) can be shortened, and the measurement error caused by the time difference can be reduced.

[0135] exist Figure 17 In the case of the second variation shown, in step 572, it is determined whether the CO2 gas concentration represented by the output signal Vout generated based on the calculated value Vout2(n), the calculated value Vout1(1), and the calculated value Vout2(0) is less than the second threshold.

[0136] Alternatively, in the second variation, a calculated value Vout1(i) (where i is an integer greater than or equal to 2) can be generated based on the detection signal Vgas12 obtained from the second or subsequent gas concentration measurement operation of the sensor unit 11 after determining that the CO2 gas concentration is above the first threshold, and the calculated value Vout1(i) can replace the calculated value Vout1(1). In this case, a calculated value Vout2(m) (where m is an integer greater than or equal to 1) can be generated based on the detection signal Vgas34 obtained from the first gas concentration measurement operation of the sensor unit 21 after the gas concentration measurement operation of the sensor unit 11 that calculates the calculated value Vout1(i), and the calculated value Vout2(m) can replace the calculated value Vout2(0).

[0137] In the second variation described above, the calculated value Vout2(0) of the detection signal Vgas34 after the calculated value Vout1(1) is obtained is used to generate the output signal Vout(n). However, instead of using the calculated value Vout2(0) of the detection signal Vgas34 after the calculated value Vout1(1) is obtained, the calculated value Vout2(1) of the second detection signal Vgas34 after the calculated value Vout1(1) can be used. Furthermore, instead of using the calculated value Vout2(0), the average value of multiple calculated values ​​Vout2(0), Vout2(1), Vout2(2)... of the multiple detection signals Vgas34 obtained after the calculated value Vout1(1) can be used. In this case, the calculated value Vout2(0) may not be included in the calculation of the average value.

[0138] Figure 19 It is used for explanation Figure 2 The flowchart of the third variation of the gas concentration measurement operation shown in (b) is as follows. Figure 20 It is used to illustrate the third variation. Figure 2 (b) shows the timing diagram of the operation of sensor units 11 and 21 during operation periods T1 and T3.

[0139] Figure 19 The action of the third variation shown is the same as Figure 16 The difference in the actions of the first variant shown is that steps 521 to 524 are added. Other basic actions are the same as... Figure 16 The actions in the first variant shown are the same, so the same symbols are assigned to the same steps, and repeated descriptions are omitted.

[0140] In the third variation, similar to the first variation, when it is determined in step 53 that the CO2 gas concentration is above the first threshold, during the period until the next gas concentration measurement operation of the sensor unit 11 is performed, the calculated value Vout1(0), the calculated value Vout2(0), and the calculated value Vout2(n) are used to generate the output signal Vout(n).

[0141] Then, when the next gas concentration measurement operation of the sensor unit 11 is executed (step 521: Yes), the calculated value Vout1 (1) is stored inside the control circuit 46 (step 522). After that, the calculated value Vout2 (0) in the second variation is replaced with the calculated value Vout2 (k) (k is an integer greater than or equal to 1), and the calculated value Vout1 (1), the calculated value Vout2 (k), and the calculated value Vout2 (n) are used to generate the output signal Vout (n) (step 523). The calculated value Vout2 (k) is equivalent to the value of the output signal Vout calculated based on the detection signal Vgas34 immediately after the initial gas concentration measurement operation of the sensor unit 11, which was determined in step 53 to be above the first threshold, has just been performed.

[0142] Then, it is determined whether the CO2 gas concentration represented by the output signal Vout generated based on the calculated value Vout2(n), the calculated value Vout1(1), and the calculated value Vout2(k) is less than the second threshold (step 524). If the current CO2 gas concentration is less than the second threshold, the gas concentration measurement operation of the sensor unit 21 is stopped (step 58), and the process returns to step 52.

[0143] That is to say, in the third variation, such as Figure 20 As shown in the timing diagram, when it is determined that the CO2 gas concentration is above the first threshold, the same operation as in the first variation is performed until the k-th gas concentration measurement operation using sensor unit 21. After the (k+1)-th gas concentration measurement operation using sensor unit 21, the same operation as in the second variation is performed. The k-th gas concentration measurement operation using sensor unit 21 refers to the concentration measurement operation using sensor unit 21 immediately after the initial gas concentration measurement operation of sensor unit 11 after determining that the CO2 gas concentration is above the first threshold in step 53. Therefore, the output signal Vout(n) can be generated in advance as in the first variation, and the measurement error can be reduced as in the second variation.

[0144] Furthermore, in the third variation, the calculated value Vout1(i) (where i is an integer greater than or equal to 2) can be generated based on the detection signal Vgas12 obtained from the second or subsequent gas concentration measurement operation of the sensor unit 11 after determining that the CO2 gas concentration is above the first threshold, and the calculated value Vout1(i) can be used to replace the calculated value Vout1(1). In this case, the calculated value Vout2(k) can also be updated based on the detection signal Vgas34 obtained from the first gas concentration measurement operation of the sensor unit 21 after the gas concentration measurement operation of the sensor unit 11 that calculates the calculated value Vout1(i).

[0145] Furthermore, in the third variation, the calculated value Vout2(0) of the detection signal Vgas34 after the calculated value Vout1(0) is obtained, and the calculated value Vout2(k) of the detection signal Vgas34 after the calculated value Vout1(1) is obtained, are used to generate the output signal Vout(n). However, instead of using the calculated value Vout2(0) of the detection signal Vgas34 after the calculated value Vout1(0), the calculated value Vout2(1) of the second detection signal Vgas34 after the calculated value Vout1(0) is obtained, and instead of using the calculated value Vout2(k) of the detection signal Vgas34 after the calculated value Vout1(1) is obtained, the calculated value Vout2(k+1) of the second detection signal Vgas34 after the calculated value Vout1(1) is obtained. Furthermore, instead of using the calculated value Vout2(0), the average of multiple calculated values ​​Vout2(0), Vout2(1), Vout2(2), ... of the multiple detection signals Vgas34 obtained after obtaining the calculated value Vout1(0) can be used. In this case, the calculated value Vout2(0) may not be included in the calculation of the average. Furthermore, instead of using the calculated value Vout2(k), the average of multiple calculated values ​​Vout2(k+1), Vout2(k+2), Vout2(k+3), ... of the multiple detection signals Vgas34 obtained after obtaining the calculated value Vout1(1) can be used. In this case, the calculated value Vout2(k+1) may not be included in the calculation of the average.

[0146] In the above example, the sensor unit 21 is not subjected to a pseudo-heating operation, but the sensor unit 21 can be subjected to a pseudo-heating operation.

[0147] Figure 21 This is a timing diagram used to illustrate the operation of the first modified example of the sensor unit 21.

[0148] like Figure 21 As shown, in the operation of the first modified example of sensor unit 21, during the gas concentration measurement operation using sensor unit 21, spurious heating of thermistors Rd3 and Rd4 is performed during the interval from the time t6 when heating of thermistors Rd3 and Rd4 stops to the time t9 when heating of thermistors Rd3 and Rd4 resumes. During the spurious heating of thermistors Rd3 and Rd4, thermistor Rd3 is heated to, for example, approximately 150°C (an example of the second temperature region), and thermistor Rd4 is heated to, for example, approximately 300°C (an example of the first temperature region). The spurious heating of thermistors Rd3 and Rd4 begins at... Figure 21The operation is performed at time t7. During the gas concentration measurement using sensor unit 21, the period from when the heating of thermistors Rd3 and Rd4 stops at time t6 to when pseudo-heating begins at time t7 is defined as the off period Toff3.

[0149] The spurious heating of thermistors Rd3 and Rd4 occurs from time t7 to time t8 after the specified on-time Ton4. The period from when the spurious heating of thermistors Rd3 and Rd4 stops at time t8 to when heating of thermistors Rd3 and Rd4 begins at time t9 is defined as the off-time Toff4.

[0150] In this way, pseudo-heating can be applied to the thermistors Rd3 and Rd4 included in the sensor section 21. Consequently, since the difference between the thermal histograms of thermistors Rd3 and Rd4 is reduced, the time-varying changes in the sensor section 21 caused by the thermal histogram difference are suppressed. To further reduce the thermal histogram difference, the length of Ton3 during the on-time period can be made the same as the length of Ton4 during the on-time period.

[0151] As explained above, in this embodiment, when the concentration of the target gas, i.e., CO2 gas, is less than the first threshold, the sensor unit 11 can be used to measure the gas concentration, which changes little and is stable over time. Furthermore, when the CO2 gas concentration is above the first threshold, by using the high-frequency gas concentration measurement of the sensor unit 21, even if the CO2 gas concentration fluctuates significantly within a short period, the output signal Vout can follow the concentration change. Moreover, when the concentration of the target gas, i.e., CO2 gas, is less than the first threshold, since the gas concentration measurement of the sensor unit 21 is stopped, power consumption during periods when the CO2 gas concentration is low and stable can be suppressed.

[0152] Furthermore, during operation T3, the gas concentration measurement operations using sensor unit 11 and sensor unit 21 can be performed asynchronously, or they can be performed at staggered times. For example, if the gas concentration measurement operations of sensor unit 11 and sensor unit 21 are staggered, it can... Figure 7 The time t20 shown is Figure 12 If the times t40 shown do not overlap, then the amplified signals Vamp1 and Vamp2 will not be input to the AD converter 44 at the same time.

[0153] exist Figures 11-20In the example shown, during the continuous gas concentration measurement operation of sensor unit 21, since no sham heating operation is performed, a thermal history difference is generated between thermistors Rd3 and Rd4, causing sensor unit 21 to change over time. For example, if control is implemented such that the cumulative execution time of the gas concentration measurement operation of sensor unit 21 during operation period T2 is shorter than the cumulative execution time of the gas concentration measurement operation of sensor unit 11 during operation period T1, the change of sensor unit 21 over time can be suppressed. During operation period T1, although the gas concentration measurement operation of sensor unit 21 is stopped, the output signal Vout can be obtained periodically because the gas concentration measurement operation and sham heating operation of sensor unit 11 are periodically repeated. To suppress the change of sensor unit 21 over time, a first threshold and a second threshold can be set such that... Figure 2 The operation period T2 shown is shorter than the operation period T1. Similarly, if control is performed such that the cumulative execution time of the gas concentration measurement operation of sensor unit 21 during operation period T3 is shorter than the cumulative execution time of the gas concentration measurement operation of sensor unit 11 during operation periods T1 and T3, the time-varying changes of sensor unit 21 can be suppressed.

[0154] Alternatively, during operation T3, the value of the output signal Vout calculated from the detection signal Vgas12 is compared with the value of the output signal Vout calculated from the detection signal Vgas34. When a non-negligible difference occurs between the two, it is determined that the output signal Vout is drifting due to residual heat in the sensor unit 21 caused by the short execution cycle C2. The first threshold and the second threshold are then changed to make the difference negligible. Alternatively, when it can be determined that the thermal history difference between thermistors Rd3 and Rd4 is increasing, and the time-varying change of the sensor unit 21 is increasing, the gas concentration measurement operation of the sensor unit 21 is prohibited. Furthermore, the drift component and the time-varying component can be removed by adding a correction to the calculation formula for calculating the output signal Vout from the detection signal Vgas34 based on the difference between the value of the output signal Vout calculated from the detection signal Vgas12 and the value of the output signal Vout calculated from the detection signal Vgas34.

[0155] Figure 22 This is a flowchart illustrating the operation of a second modified example of the sensor unit 21.

[0156] exist Figure 22In the second variation shown, similar to the first variation, the sensor unit 21 performs not only gas concentration measurement but also pseudo-heating. When performing gas concentration measurement using the sensor unit 21, firstly, 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 starts heating the thermistors Rd3 and Rd4 by outputting the heater indication value calculated based on the ambient temperature to the DA converter 45 (step 302). The heater indication value is converted by the DA converter 45 into heater voltages Vmh3 and Vmh4, which are applied to heaters MH3 and MH4, respectively. In step 302, thermistor Rd3 is heated to, for example, about 300°C, and thermistor Rd4 is heated to, for example, about 150°C (when the CO2 gas concentration in the measuring atmosphere is, for example, zero, thermistor Rd3 is heated to, for example, 300°C, and thermistor Rd4 is heated to, for example, 150°C).

[0157] After a predetermined preparation time, the signal processing circuit 40 samples the detection signal Vgas34 (step 303). Then, the signal processing circuit 40 calculates the output signal Vout from the detection signal Vgas34 and outputs the output signal Vout to the outside. Next, the control circuit 46 stops the heating of thermistors Rd3 and Rd4 by resetting the heater indication value (step 304). In the gas concentration measurement operation using the sensor unit 21, the period from the start of heating of thermistors Rd3 and Rd4 in step 302 to the stop of heating of thermistors Rd3 and Rd4 in step 304 is defined as the on-time Ton3.

[0158] Next, the control circuit 46 determines whether to stop the gas concentration measurement operation of the sensor unit 21 (step 305). Whether to stop the gas concentration measurement operation of the sensor unit 21 is determined based on whether the CO2 gas concentration measured by at least one of the sensor units 11 and 21 is less than a second threshold. This determination can be based on the condition that both the CO2 gas concentrations measured by the sensor units 11 and 21 are less than the second threshold, or it can be based on the condition that either the CO2 gas concentration measured by the sensor units 11 and 21 is less than the second threshold. Then, if it is determined that the gas concentration measurement operation of the sensor unit 21 should not be stopped, the circuit returns to step 301 to calculate the ambient temperature and then begins reheating of the thermistors Rd3 and Rd4. During the gas concentration measurement operation using the sensor unit 21, the period from the cessation of heating of the thermistors Rd3 and Rd4 to the resumption of heating is defined as the off-time Toff5. On the other hand, if it is determined that the gas concentration measurement operation of the sensor unit 21 is to be stopped, the control circuit 46 stores the cumulative heating time of the thermistors Rd3 and Rd4 during the gas concentration measurement operation of the sensor unit 21 (cumulative time of Ton3 during the on-time) (step 306), stops the gas concentration measurement operation of the sensor unit 21, and starts the pseudo heating operation of the sensor unit 21 (step 307).

[0159] In the pseudo-heating operation of the sensor unit 21, firstly, pseudo-heating of thermistors Rd3 and Rd4 is initiated (step 308). During the pseudo-heating of thermistors Rd3 and Rd4, thermistor Rd3 is heated to, for example, approximately 150°C, and thermistor Rd4 is heated to, for example, approximately 300°C. Next, the control circuit 46 stops the heating of thermistors Rd3 and Rd4 (step 309). In the pseudo-heating operation of the sensor unit 21, 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-state period Ton4. The length of the on-state period Ton4 can be the same as the length of the on-state period Ton3.

[0160] Next, the control circuit 46 determines whether the cumulative heating time (cumulative time of Ton4 during the on-state) of thermistors Rd3 and Rd4 during the pseudo-heating operation of sensor unit 21 is less than the cumulative heating time (cumulative time of Ton3 during the on-state) of thermistors Rd3 and Rd4 during the gas concentration measurement operation of sensor unit 21 (step 310). If the determination result is that the cumulative time of Ton4 during the on-state is less than the cumulative time of Ton3 during the on-state, that is, if the cumulative heating time during the pseudo-heating operation of sensor unit 21 is less than the cumulative heating time during the gas concentration measurement operation of sensor unit 21, the control circuit 46 determines whether to stop the pseudo-heating operation of sensor unit 21 (step 311). Whether to stop the pseudo-heating operation of sensor unit 21 can be determined based on whether the CO2 gas concentration measured by sensor unit 11 is above a first threshold. Then, if it is determined that the pseudo-heating operation of sensor unit 21 should not be stopped, the circuit returns to step 308 and starts pseudo-heating of thermistors Rd3 and Rd4 again. In the pseudo-heating operation of the sensor unit 21, the period from the cessation of pseudo-heating of thermistors Rd3 and Rd4 to the start of pseudo-heating is defined as the off period Toff6.

[0161] On the other hand, if in step 310 it is determined that the cumulative time of Ton4 during the on-time period is greater than or equal to the cumulative time of Ton3 during the on-time period, that is, if it is determined that the cumulative heating time during the pseudo-heating operation of the sensor unit 21 is greater than or equal to the cumulative heating time during the gas concentration measurement operation of the sensor unit 21, or if it is determined in step 311 that the pseudo-heating operation of the sensor unit 21 should be stopped, the control circuit 46 stores the cumulative heating time of the thermistors Rd3 and Rd4 during the pseudo-heating operation of the sensor unit 21 (the cumulative time of Ton4 during the on-time period) (step 312), and stops the pseudo-heating operation of the sensor unit 21 (step 313).

[0162] Figure 23 This is a timing diagram illustrating an example of the operation of a second modified version of the sensor unit 21.

[0163] exist Figure 23 In the example shown, after the gas concentration measurement operation using sensor unit 21 is performed in period 401, a pseudo-heating operation of sensor unit 21 is performed in period 402. Period 401 starts in response to the CO2 gas concentration measured by sensor unit 11 being above a first threshold, and ends in response to the CO2 gas concentration measured by at least one of sensor units 11 and 21 being below a second threshold. On the other hand, period 402 can start at any time after period 401 has ended, after any preparation time has elapsed. Furthermore, the lengths of the opening period Ton3 and the opening period Ton4 do not need to be the same, such as... Figure 24 As shown in the example, the length of the on-time Ton4 can be set to be longer than the length of the on-time Ton3. In this case, step 308 can continue until the cumulative heating time (cumulative time of the on-time Ton4) during the pseudo-heating operation of the sensor unit 21 reaches the cumulative heating time (cumulative time of the on-time Ton3) during the gas concentration measurement operation of the sensor unit 21.

[0164] Figure 25 This is a circuit diagram illustrating the structure of a gas sensor 200 according to a second embodiment of the technology disclosed herein.

[0165] like Figure 25 As shown, the gas sensor 200 of the second embodiment differs from the gas sensor 100 of the first embodiment in that: sensor section 11 is replaced by sensor section 12; sensor section 21 is replaced by sensor section 22; and differential amplifiers 41 and 42 included in the signal processing circuit 40 are replaced by differential amplifiers 47 and 48, respectively. Other basic configurations are the same as those of the gas sensor 100 of the first embodiment; therefore, the same reference numerals are used for the same elements, and repeated descriptions are omitted.

[0166] The sensor unit 12 includes: a thermistor Rd1 and a fixed resistor R1 connected in series between the power supply Vcc and the ground GND; a thermistor Rd2 and a fixed resistor R2 connected in series between the power supply Vcc and the ground GND; and heaters MH1 and MH2. The temperature of thermistor Rd1 changes accordingly with the temperature change of heater MH1, and the temperature of thermistor Rd2 changes accordingly with the temperature change of heater MH2. A detection signal Vgas1 appears at the connection point N1 of the thermistor Rd1 and the fixed resistor R1. A detection signal Vgas2 appears at the connection point N2 of the thermistor Rd2 and the fixed resistor R2.

[0167] The sensor unit 22 includes: a thermistor Rd3 and a fixed resistor R3 connected in series between the power supply Vcc and the ground GND; a thermistor Rd4 and a fixed resistor R4 connected in series between the power supply Vcc and the ground GND; and heaters MH3 and MH4. The temperature of thermistor Rd3 changes accordingly with the temperature change of heater MH3, and the temperature of thermistor Rd4 changes accordingly with the temperature change of heater MH4. A detection signal Vgas3 appears at the connection point N3 of the thermistor Rd3 and the fixed resistor R3. A detection signal Vgas4 appears at the connection point N4 of the thermistor Rd4 and the fixed resistor R4.

[0168] Differential amplifier 47 compares detection signals Vgas1 and Vgas2 to generate an amplified signal Vamp1, which is the level difference between detection signals Vgas1 and Vgas2 (=Vgas2-Vgas1). Differential amplifier 48 compares detection signals Vgas3 and Vgas4 to generate an amplified signal Vamp2, which is the level difference between detection signals Vgas3 and Vgas4 (=Vgas4-Vgas3).

[0169] As illustrated in the gas sensor 200 of the second embodiment, it is not necessary to connect thermistors Rd1 and Rd2 in series. Alternatively, thermistors Rd1 and Rd2 can be connected in parallel between the power supply Vcc and ground GND, and the amplified signal Vamp1 can be generated based on the difference between the output voltage (detection signal Vgas1) caused by thermistor Rd1 and the output voltage (detection signal Vgas2) caused by thermistor Rd2. Similarly, it is not necessary to connect thermistors Rd3 and Rd4 in series. Alternatively, thermistors Rd3 and Rd4 can be connected in parallel between the power supply Vcc and ground GND, and the amplified signal Vamp2 can be generated based on the difference between the output voltage (detection signal Vgas3) caused by thermistor Rd3 and the output voltage (detection signal Vgas4) caused by thermistor Rd4.

[0170] The above describes the implementation of the technology disclosed herein. However, the technology disclosed herein is not limited to the above implementation. Various modifications can be made without departing from its spirit. Needless to say, these modifications are also included within the scope of the technology disclosed herein.

[0171] For example, in the above embodiments, a resistive element, namely a thermistor, is used as the temperature sensing element of the sensor sections 11, 12, 21, and 22, but the present invention is not limited thereto. For example, a resistive element, namely platinum (Pt) or tungsten (W), can be used as the temperature sensing element.

[0172] Furthermore, in the above embodiment, the sensor unit 11 is pseudo-heated, but if the effect of time change is small, the pseudo-heating of the sensor unit 11 may not be performed.

[0173] The technology disclosed herein includes, but is not limited to, the following configuration examples.

[0174] A gas sensor according to one aspect of this disclosure includes: a first sensor unit that outputs a first detection signal corresponding to the concentration of a target gas; a second sensor unit that outputs a second detection signal corresponding to the concentration of the target gas; and 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. The signal processing circuit repeatedly executes a first gas concentration measurement operation by controlling the first sensor unit to acquire the first detection signal. In response to a gas concentration calculated from the first detection signal becoming a first threshold, a second gas concentration measurement operation by controlling the second sensor unit to acquire the second detection signal is started and repeatedly executed. In response to at least one of the gas concentration calculated from the first detection signal and at least the gas concentration calculated from the second detection signal being less than a second threshold, the second gas concentration measurement operation is stopped. The number of times the second gas concentration measurement operation is executed per unit time during the period from the start to the stop of the second gas concentration measurement operation is greater than the number of times the first gas concentration measurement operation is executed per unit time during the period of repeatedly executing the first gas concentration measurement operation. Therefore, while suppressing power consumption, the measurement result can follow the concentration change even when the concentration of the target gas changes significantly in a short period of time.

[0175] In the aforementioned gas sensor, the first sensor unit may include a first temperature-sensing element and a second temperature-sensing element, a first heater for heating the first temperature-sensing element, and a second heater for heating the second temperature-sensing element; the second sensor unit may include a third temperature-sensing element and a fourth temperature-sensing element, a third heater for heating the third temperature-sensing element, and a fourth heater for heating the fourth temperature-sensing element. When controlling the first sensor unit, the signal processing circuit alternately and repeatedly executes: a first gas concentration measurement operation where the first temperature-sensing element is heated to a first temperature region by the first heater, and the second temperature-sensing element is heated to a second temperature region by the second heater; and a first pseudo-heating operation where the first temperature-sensing element is heated to the second temperature region by the first heater, and the second temperature-sensing element is heated to the first temperature region by the second heater. When controlling the second sensor unit, a second gas concentration measurement operation where the third temperature-sensing element is heated to the first temperature region by the third heater, and the fourth temperature-sensing element is heated to the second temperature region by the fourth heater is repeatedly executed. This suppresses the time-varying changes of the first sensor unit.

[0176] In the aforementioned gas sensor, the first temperature region may be higher than the second temperature region. The interval between the action of heating the first temperature-sensing element to the first temperature region via the first heater during the first gas concentration measurement action and the action of heating the first temperature-sensing element to the second temperature region via the first heater during the first spurious heating action is longer than the interval between the action of heating the second temperature-sensing element to the second temperature region via the second heater during the first gas concentration measurement action and the action of heating the second temperature-sensing element to the first temperature region via the second heater during the first spurious heating action. This reduces the thermal history difference between the first and second temperature-sensing elements.

[0177] In the aforementioned gas sensor, the interval between the first spurious heating action and the first gas concentration measurement action can be more than 10 times the execution time of the spurious heating action. This reduces the measurement error of the first sensor unit caused by residual heat.

[0178] In the aforementioned gas sensor, the signal processing circuit, when controlling the second sensor unit, may repeatedly perform the second gas concentration measurement operation without executing the heating operation of heating the third temperature sensing element to the second temperature region via the third heater and the fourth temperature sensing element to the first temperature region via the fourth heater. This allows for easy high-frequency execution of the second gas concentration measurement operation.

[0179] In the aforementioned gas sensor, the signal processing circuit, while controlling the second sensor unit, may also perform a second pseudo-heating operation: heating the third temperature-sensing element to the second temperature region via a third heater, and heating the fourth temperature-sensing element to the first temperature region via a fourth heater. This suppresses changes in the second sensor unit over time. In this case, the signal processing circuit can alternately and repeatedly perform the second gas concentration measurement operation and the second pseudo-heating operation while controlling the second sensor unit. This simplifies the control of the second sensor unit.

[0180] In the aforementioned gas sensor, a first temperature-sensing element and a second temperature-sensing element may be connected in series, and a third temperature-sensing element and a fourth temperature-sensing element may be connected in series. When the first gas concentration measurement is activated, the signal processing circuit obtains a first detection signal from the connection point of the first and second temperature-sensing elements; when the second gas concentration measurement is activated, it obtains a second detection signal from the connection point of the third and fourth temperature-sensing elements. This simplifies the circuit structure of the first and second sensor units.

[0181] In the aforementioned gas sensor, the signal processing circuit can execute the first gas concentration measurement operation and the second gas concentration measurement operation in parallel. Therefore, while executing the second gas concentration measurement operation, the concentration of the target gas can also be measured through the first gas concentration measurement operation.

[0182] In the aforementioned gas sensor, the cumulative execution time of the second gas concentration measurement action can be shorter than the cumulative execution time of the first gas concentration measurement action. This allows for the suppression of time-dependent changes in the second sensor unit.

[0183] In the aforementioned gas sensor, the signal processing circuit can calculate the concentration of the target gas based on the second detection signal and the first detection signal during the second gas concentration measurement operation. Therefore, even if there are differences in characteristics between the first and second sensor units, measurement errors can be reduced.

[0184] In the gas sensor described above, the signal processing circuit can calculate the concentration of the target gas during the second gas concentration measurement operation by adding a first detection value based on a first detection signal to a third detection value based on the latest second detection signal and subtracting a second detection value based on the second detection signal obtained after obtaining the first detection value.

Claims

1. A gas sensor, characterized in that, include: The first sensor unit outputs a first detection signal corresponding to the concentration of the gas to be detected; The second sensor unit outputs a second detection signal corresponding to the concentration of the gas to be detected; and The signal processing circuit controls the first sensor unit and the second sensor unit, and calculates the concentration of the target gas based on the first detection signal and the second detection signal. The signal processing circuit, The first gas concentration measurement action, which acquires the first detection signal by controlling the first sensor unit, is repeatedly executed. In response to the gas concentration calculated from the first detection signal becoming a first threshold or higher, a second gas concentration measurement operation is initiated and repeatedly executed by controlling the second sensor unit to acquire the second detection signal. In response to at least one of the gas concentration calculated from the first detection signal and the gas concentration calculated from the second detection signal being less than a second threshold, the second gas concentration measurement operation is stopped. The number of times the second gas concentration measurement action is executed per unit time during the period from the start to the stop of the second gas concentration measurement action is greater than the number of times the first gas concentration measurement action is executed per unit time during the period of repeatedly executing the first gas concentration measurement action.

2. The gas sensor according to claim 1, characterized in that, The first sensor unit includes a first temperature sensing element and a second temperature sensing element, a first heater for heating the first temperature sensing element, and a second heater for heating the second temperature sensing element. The second sensor unit includes a third temperature sensing element and a fourth temperature sensing element, a third heater for heating the third temperature sensing element, and a fourth heater for heating the fourth temperature sensing element. When controlling the first sensor unit, the signal processing circuit alternately and repeatedly executes the first gas concentration measurement operation, which heats the first temperature sensing element to a first temperature region via the first heater and the second temperature sensing element to a second temperature region via the second heater, and the first pseudo-heating operation, which heats the first temperature sensing element to the second temperature region via the first heater and the second temperature sensing element to the first temperature region via the second heater. When controlling the second sensor unit, the circuit repeatedly executes the second gas concentration measurement operation, which heats the third temperature sensing element to the first temperature region via the third heater and the fourth temperature sensing element to the second temperature region via the fourth heater.

3. The gas sensor according to claim 2, characterized in that, The first temperature region is hotter than the second temperature region. The interval between the action 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 action of heating the first temperature sensing element to the second temperature region by the first heater in the first pseudo-heating operation is longer than the interval between the action 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 action of heating the second temperature sensing element to the first temperature region by the second heater in the first pseudo-heating operation.

4. The gas sensor according to claim 2, characterized in that, The interval between the first spurious heating action and the first gas concentration measurement action is more than 10 times the execution time of the spurious heating action.

5. The gas sensor according to claim 2, characterized in that, When controlling the second sensor unit, the signal processing circuit repeats the second gas concentration measurement operation without performing the heating operation of heating the third temperature sensing element to the second temperature region through the third heater and heating the fourth temperature sensing element to the first temperature region through the fourth heater.

6. The gas sensor according to claim 2, characterized in that, When controlling the second sensor unit, the signal processing circuit also performs a second pseudo-heating operation, which heats the third temperature sensing element to the second temperature region via the third heater and the fourth temperature sensing element to the first temperature region via the fourth heater.

7. The gas sensor according to claim 6, characterized in that, When controlling the second sensor unit, the signal processing circuit alternately and repeatedly executes the second gas concentration measurement action and the second pseudo-heating action.

8. The gas sensor according to any one of claims 2 to 7, characterized in that, The first temperature sensing element and the second temperature sensing element are connected in series. The third temperature sensing element is connected in series with the fourth temperature sensing element. When the first gas concentration measurement is activated, the signal processing circuit obtains the first detection signal from the connection point between the first temperature sensing element and the second temperature sensing element; when the second gas concentration measurement is activated, it obtains the second detection signal from the connection point between the third temperature sensing element and the fourth temperature sensing element.

9. The gas sensor according to claim 1, characterized in that, The signal processing circuit executes the first gas concentration measurement action and the second gas concentration measurement action in parallel.

10. The gas sensor according to claim 1, characterized in that, The cumulative execution time of the second gas concentration measurement action is shorter than the cumulative execution time of the first gas concentration measurement action.

11. The gas sensor according to claim 1, characterized in that, During the second gas concentration measurement operation, the signal processing circuit calculates the concentration of the target gas based on the second detection signal and the first detection signal.

12. The gas sensor according to claim 11, characterized in that, In the second gas concentration measurement operation, the signal processing circuit calculates the concentration of the target gas by adding a first detection value based on the first detection signal to a third detection value based on the latest second detection signal and subtracting a second detection value based on the second detection signal obtained after obtaining the first detection value.

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