Gas sensor

Abstract

Provided is a gas sensor capable of accurately measuring gas concentration regardless of the flow rate of gas in a measured atmosphere. A gas sensor (100) is provided with: a sensor unit (10) that generates a detection signal (Vgas) corresponding to the concentration of a gas to be detected; and a control circuit (35) that calculates an output signal (Vout) indicating the concentration of the gas to be detected on the basis of the detection signal (Vgas). The sensor unit (10) includes thermistors (11, 12) and heaters (13, 14) that heat the thermistors (11, 12). The control circuit (35) corrects the output signal (Vout) on the basis of a flow rate signal (S) indicating the gas flow rate in the measured atmosphere. The gas sensor (200, 300) changes the heating condition of the heater (13, 14) according to the flow rate signal S.

Description

Technical Field

[0001] The present disclosure relates to a gas sensor, and more particularly to a gas sensor capable of accurately measuring gas concentration regardless of the flow rate of a measurement atmosphere. Background Art

[0002] Patent Document 1 discloses a gas sensor capable of reducing measurement errors caused by a gas different from a detection target gas.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: Japanese Patent No. 7070175 Summary of the Invention

[0006] Technical problem to be solved by the invention

[0007] The present inventors have discovered that measurement errors occur due to the flow rate of gas in the measurement atmosphere.

[0008] The present disclosure describes a technique related to a gas sensor capable of accurately measuring gas concentration regardless of the gas flow rate in the measurement atmosphere.

[0009] Technical solutions to technical problems

[0010] One aspect of the present disclosure relates to a gas sensor comprising: a sensor portion that generates a detection signal corresponding to the concentration of the detection target gas; and a control circuit that calculates an output signal representing the concentration of the detection target gas based on the detection signal, wherein the sensor portion includes a temperature sensing element and a heater for heating the temperature sensing element, and the control circuit corrects the output signal or changes the heating conditions of the heater based on a flow rate signal representing the gas flow rate in the measurement atmosphere.

[0011] Effects of the Invention

[0012] According to the present disclosure, a gas sensor capable of accurately measuring gas concentration regardless of the gas flow rate in a measurement atmosphere is provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 1 is a circuit diagram showing the configuration of a gas sensor 100 according to the first embodiment of the technology disclosed herein.

[0014] Figure 2 This is a circuit example of the reference voltage generating circuit 32 .

[0015] Figure 3 This is a graph for explaining the influence of the gas flow rate in the measurement atmosphere on the measurement results.

[0016] Figure 4 1 is a circuit diagram showing the configuration of a gas sensor 200 according to a second embodiment of the technology disclosed herein.

[0017] Figure 5 This is a timing chart for explaining a correction method of the heater voltages V13 and V14 using the heater voltage correction table 35 b.

[0018] Figure 6 1 is a circuit diagram showing the structure of a gas sensor 300 according to a third embodiment of the technology disclosed herein.

[0019] Figure 7 This is a timing chart for explaining a method of correcting the heating time using the heating time correction table 35c.

[0020] Explanation of symbols:

[0021] 10Sensor unit

[0022] 11, 12 thermistor

[0023] 13, 14 heater

[0024] 20 temperature sensors

[0025] 21 resistors

[0026] 22 Thermistor

[0027] 30 signal processing circuit

[0028] 31 multiplexer

[0029] 32 reference voltage generation circuit

[0030] 32a DA converter

[0031] 33 Differential Amplifier

[0032] 34AD converter

[0033] 35 control circuit

[0034] 35a concentration correction table

[0035] 35b heater voltage correction table

[0036] 35°C heating time correction table

[0037] 36 drive circuit

[0038] 40 flow rate sensor

[0039] 100, 200, 300 gas sensors

[0040] N1, N2 connection points

[0041] S flow rate signal

[0042] VR1, VR2 variable resistors

[0043] Vamp amplifies the signal

[0044] Vgas detection signal

[0045] Vout output signal

[0046] Vref reference signal

[0047] Vtemp temperature signal DETAILED DESCRIPTION

[0048] Below, with reference to the attached Figure 1 Embodiments of the technology disclosed herein will be described in detail.

[0049] <First embodiment>

[0050] Figure 1 1 is a circuit diagram showing the configuration of a gas sensor 100 according to the first embodiment of the technology disclosed herein.

[0051] like Figure 1 As shown, the gas sensor 100 of the first embodiment includes a sensor unit 10 that generates a detection signal Vgas corresponding to the concentration of the detection target gas, a temperature sensor 20 that generates a temperature signal Vtemp corresponding to the ambient temperature, and a signal processing circuit 30. Although not particularly limited, the gas sensor 100 of this embodiment is a thermal conduction gas sensor for detecting the concentration of CO2 gas in the measurement atmosphere.

[0052] The sensor unit 10 includes thermistors 11 and 12 connected in series between a power supply Vcc and a ground GND, and heaters 13 and 14 for heating thermistors 11 and 12, respectively. A detection signal Vgas output from the sensor unit 10 appears at a connection point N1 between thermistors 11 and 12. Thermistor 11 is a temperature-sensing element for detection, and thermistor 12 is a temperature-sensing element for reference. Thermistors 11 and 12 are resistors whose resistance value changes with temperature. Examples of materials for thermistors 11 and 12, as well as thermistor 22 described later, include vanadium oxide, amorphous silicon, polycrystalline silicon, an oxide having a spinel crystal structure containing manganese, titanium oxide, or yttrium-barium-copper oxide.

[0053] When thermistor 11, serving as a temperature-sensing element for detection, is heated to a temperature range of 100°C to 230°C, for example, 150°C, which is a temperature range with high CO2 gas detection sensitivity, and CO2 gas is present in the measurement atmosphere, the heat dissipation characteristics of thermistor 11 change depending on the CO2 gas concentration. This change manifests as a change in the temperature of thermistor 11, or in other words, a change in the resistance value of thermistor 11. Specifically, CO2 gas has lower heat dissipation properties than air. Therefore, the higher the CO2 gas concentration, the higher the temperature of thermistor 11. Therefore, if thermistor 11 is heated to 150°C when the CO2 gas concentration in the measurement atmosphere is zero, for example, the temperature of thermistor 11 will rise above 150°C when CO2 gas is present in the measurement atmosphere, depending on the CO2 gas concentration. As a result, the resistance value of thermistor 11 decreases as the CO2 gas concentration in the measurement atmosphere increases.

[0054] On the other hand, even when CO₂ gas is present in the measurement atmosphere while thermistor 12, serving as a reference temperature sensing element, is heated to a temperature range of 300°C to 450°C, for example, near 300°C, which is a temperature range with low CO₂ gas detection sensitivity, the heat dissipation characteristics of thermistor 12 barely change depending on the CO₂ gas concentration, and the temperature of thermistor 12 barely changes. Therefore, the change in resistance value of thermistor 12 due to CO₂ gas concentration when heated to near 300°C is much smaller than the change in resistance value of thermistor 11 due to CO₂ gas concentration when heated to near 150°C. The change in resistance value of thermistor 12 due to CO₂ gas concentration when heated to near 300°C can also be virtually nonexistent. As a result, when thermistor 11 is heated to approximately 150°C and thermistor 12 is heated to approximately 300°C (when the CO2 gas concentration in the measurement atmosphere is, for example, zero, thermistor 11 is heated to 150°C and thermistor 12 is heated to 300°C), a detection signal Vgas corresponding to the CO2 gas concentration in the measurement atmosphere appears at the connection point N1 between thermistors 11 and 12. On the other hand, even if the measurement atmosphere contains another gas whose heat dissipation characteristics when thermistor 11 is heated to approximately 150°C and when thermistor 12 is heated to approximately 300°C do not differ significantly, the concentration of that gas has little effect on the detection signal Vgas. Thus, sensor unit 10 can selectively detect the concentration of CO2 gas.

[0055] The temperature sensor 20 includes a resistor 21 and a thermistor 22 connected in series between a power supply Vcc and a ground GND. A temperature signal Vtemp from the temperature sensor 20 appears at a connection point N2 between the resistor 21 and the thermistor 22. The temperature sensor 20 detects the ambient temperature, which is the temperature of the measurement atmosphere. For example, the temperature sensor 20 can be designed to be unaffected or less susceptible to the heating generated by the heaters 13 and 14.

[0056] The signal processing circuit 30 includes a multiplexer 31 , a reference voltage generating circuit 32 , a differential amplifier 33 , an AD converter (ADC) 34 , a control circuit 35 , and a drive circuit 36 ​​.

[0057] The multiplexer 31 supplies one of the detection signal Vgas and the temperature signal Vtemp to the differential amplifier 33 under the control of the control circuit 35. The differential amplifier 33 generates an amplified signal Vamp, which is obtained by amplifying the level difference (potential difference) between the level of one of the detection signal Vgas and the temperature signal Vtemp and the level of the reference signal Vref generated by the reference voltage generation circuit 32. The reference voltage generation circuit 32 can be as follows: Figure 2 As shown in (a), the DA converter 32a performs DA conversion on the digital value output from the control circuit 35, and can also be configured as shown in (a). Figure 2 (b) is composed of variable resistors VR1 and VR2 whose resistance values ​​are controlled by a control circuit 35.

[0058] The amplified signal Vamp output from the differential amplifier 33 is input to the AD converter 34 . The AD converter 34 generates a digital value by performing AD conversion on the amplified signal Vamp, and supplies the generated digital value to the control circuit 35 .

[0059] The control circuit 35 calculates the concentration of CO2 gas, the target gas for detection, based on the amplified signal Vamp obtained by amplifying the detection signal Vgas, and generates an output signal Vout indicating the concentration of CO2 gas. The CO2 gas concentration is calculated using a calculation formula set within the control circuit 35. Furthermore, the control circuit 35 controls the levels of the heater voltages V13 and V14 supplied to the heaters 13 and 14, respectively, via the drive circuit 36.

[0060] Control circuit 35 corrects heater voltages V13 and V14 based on amplified signal Vamp, which is amplified from temperature signal Vtemp. When the gas flow rate in the measurement atmosphere is zero and the CO2 gas concentration in the measurement atmosphere is, for example, zero, control circuit 35 corrects heater voltages V13 and V14 so that the temperatures of thermistors 11 and 12 reach 150°C and 300°C, respectively, through heating by heaters 13 and 14 for a predetermined period of time, regardless of the ambient temperature. Specifically, control circuit 35 changes the levels of heater voltages V13 and V14 based on temperature signal Vtemp (amplified signal Vamp), thereby varying the power applied to heaters 13 and 14 and, therefore, varying the amount of heat generated by heaters 13 and 14.

[0061] The control circuit 35 also corrects the output signal Vout based on the flow rate signal S supplied from the flow rate sensor 40. The flow rate sensor 40 may be part of the gas sensor 100 or a device external to the gas sensor 100. The output signal Vout is corrected by referring to a concentration correction table 35a set within the control circuit 35. The concentration correction table 35a is a data table that shows the relationship between the flow rate signal S and the required correction amount for the output signal Vout. The flow rate signal S represents the gas flow rate in the measurement atmosphere. The higher the flow rate in the flow rate signal S, the lower the heating temperature of the thermistors 11 and 12. Therefore, the control circuit 35 corrects this.

[0062] Figure 3 This is a graph for explaining the influence of the gas flow rate in the measurement atmosphere on the measurement results, and shows the relationship between the heating temperature and the detection sensitivity of the thermistors 11 and 12.

[0063] like Figure 3 As shown, the detection sensitivity of thermistors 11 and 12, that is, the relationship between the CO2 gas concentration in the measurement atmosphere and the resistance values ​​of thermistors 11 and 12, varies significantly depending on the heating temperature of thermistors 11 and 12. Specifically, the CO2 gas detection sensitivity of thermistors 11 and 12 is maximum at approximately 150°C, while being substantially zero in the temperature range above 300°C. Therefore, as described above, by heating thermistor 11 to approximately 150°C and thermistor 12 to approximately 300°C, the CO2 gas concentration can be selectively detected.

[0064] However, when the gas flows in the measurement atmosphere, the thermistors 11 and 12 are cooled by the flow of gas, and the heating temperature of the thermistors 11 and 12 is reduced. Therefore, the greater the flow rate, the lower the heating temperature of the thermistor 11. Figure 3As shown by the arrow A, the lower the temperature, the lower the detection sensitivity. As a result, the change in the detection signal Vgas relative to the change in CO2 gas concentration becomes smaller. In addition, the greater the flow rate, the lower the heating temperature of the thermistor 12. Figure 3 As indicated by arrow B, the temperature decreases as the heating temperature falls below 300°C, and the detection sensitivity increases. Consequently, the resistance value of thermistor 12 changes with the CO2 gas concentration, and the amount of change in detection signal Vgas relative to changes in CO2 gas concentration decreases. Thus, when gas flows in the measurement atmosphere, detection signal Vgas changes according to the flow rate.

[0065] Control circuit 35 uses concentration correction table 35a to correct output signal Vout to eliminate measurement errors caused by this flow velocity. The amount of correction applied to output signal Vout is determined by flow velocity signal S. The greater the flow velocity indicated by flow velocity signal S, the greater the correction applied to output signal Vout. This allows accurate CO2 gas concentration detection regardless of the flow velocity of the measurement atmosphere.

[0066] Furthermore, the control circuit 35 can also change the level of the reference signal Vref according to the flow rate signal S. Thus, even if the midpoint level of the detection signal Vgas (the level of the detection signal Vgas appearing at the connection point N1 when the CO2 gas concentration in the measurement atmosphere is, for example, zero) is offset due to the gas flow rate in the measurement atmosphere, the offset can be canceled, thereby preventing a reduction in the dynamic range.

[0067] As described above, the gas sensor 100 according to the first embodiment corrects the output signal Vout based on the flow rate signal S. Therefore, the concentration of CO 2 gas can be accurately detected regardless of the gas flow rate in the measurement atmosphere.

[0068] The measurement error caused by the gas flow rate in the measurement atmosphere can also be corrected by changing the heating conditions of the heaters 13 and 14. An embodiment of correcting the measurement error caused by the flow rate by changing the heating conditions of the heaters 13 and 14 will be described below.

[0069] <Second embodiment>

[0070] Figure 4 1 is a circuit diagram showing the configuration of a gas sensor 200 according to a second embodiment of the technology disclosed herein.

[0071] like Figure 4As shown, the gas sensor 200 of the second embodiment differs from the gas sensor 100 of the first embodiment in that the control circuit 35 includes a heater voltage correction table 35b instead of the concentration correction table 35a. The rest of the basic structure is the same as that 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.

[0072] The control circuit 35 uses a heater voltage correction table 35b to correct the levels of the heater voltages V13 and V14 to eliminate measurement errors caused by the gas flow rate in the measurement atmosphere. The heater voltage correction table 35b is a data table that shows the relationship between the flow rate signal S and the correction amount of the heater voltages V13 and V14.

[0073] Figure 5 This is a timing chart for explaining a correction method of the heater voltages V13 and V14 using the heater voltage correction table 35 b.

[0074] exist Figure 5 In the illustrated example, heater voltages V13 and V14 are applied to the heaters 13 and 14 , respectively, during a period T1 between time t1 and time t3 . Figure 5 The level V13a shown is the level of the heater voltage V13 when the flow rate of the gas in the measurement atmosphere is zero. Figure 5 Level V14a shown is the level of heater voltage V14 when the flow rate of the gas in the measurement atmosphere is zero. Specifically, when the flow rate of the gas in the measurement atmosphere is zero, and the levels of heater voltages V13 and V14 are set to V13a and V14a, respectively, the heating temperatures of thermistors 11 and 12 become approximately 150°C and approximately 300°C, respectively. (For example, when the CO2 gas concentration in the measurement atmosphere is zero, the heating temperatures of thermistors 11 and 12 become 150°C and 300°C, respectively.)

[0075] However, if gas flow occurs in the measurement atmosphere, setting the levels of heater voltages V13 and V14 to V13a and V14a, respectively, cools thermistors 11 and 12 due to the gas flow, causing the heating temperatures of thermistors 11 and 12 to drop to 150°C -α and 300°C -β, respectively. Control circuit 35 sets the levels of heater voltages V13 and V14 to V13b (> V13a) and V14b (> V14a), respectively, based on flow rate signal S, to eliminate this drop in heating temperature. Corrected level V13b of heater voltage V13 is obtained by adding a correction amount corresponding to flow rate signal S from heater voltage correction table 35b to level V13a. Corrected level V14b of heater voltage V14 is obtained by adding a correction amount corresponding to flow rate signal S from heater voltage correction table 35b to level V14a. Thus, even when gas flows in the measurement atmosphere, thermistors 11 and 12 are accurately heated to approximately 150° C. and 300° C., respectively. Furthermore, sampling detection signal Vgas at time t2 before time t3 allows accurate measurement of gas concentration.

[0076] In this manner, the gas sensor 200 of the second embodiment changes the levels of heater voltages V13 and V14 based on flow velocity signal S, thereby varying the power applied to heaters 13 and 14 and, consequently, the amount of heat generated by heaters 13 and 14. The correction amount for heater voltages V13 and V14 is determined by flow velocity signal S. The higher the flow velocity indicated by flow velocity signal S, the greater the correction amount for heater voltages V13 and V14. This allows accurate CO2 gas concentration detection regardless of the flow velocity of the measurement atmosphere.

[0077] <Third embodiment>

[0078] Figure 6 1 is a circuit diagram showing the structure of a gas sensor 300 according to a third embodiment of the technology disclosed herein.

[0079] like Figure 6 As shown in FIG. 1 , the gas sensor 300 of the third embodiment differs from the gas sensor 100 of the first embodiment in that a heating time correction table 35c is included in the control circuit 35 instead of the concentration correction table 35a. The rest of the basic structure is the same as that of the gas sensor 100 of the first embodiment. Therefore, the same reference numerals are used to designate the same elements, and duplicate descriptions are omitted.

[0080] The control circuit 35 uses a heating time correction table 35c to correct the application time of the heater voltages V13 and V14 to eliminate measurement errors caused by the gas flow rate in the measurement atmosphere. The heating time correction table 35c is a data table that shows the relationship between the flow rate signal S and the application time of the heater voltages V13 and V14.

[0081] Figure 7 This is a timing chart for explaining a method of correcting the heating time using the heating time correction table 35c.

[0082] exist Figure 7 In the example shown, when the flow rate of the gas in the measurement atmosphere is zero, heater voltages V13 and V14 are applied to heaters 13 and 14, respectively, during a period T1 between time t1 and time t3. When the flow rate of the gas in the measurement atmosphere is zero, the heating temperatures of thermistors 11 and 12 reach approximately 150°C and approximately 300°C, respectively, at time t2, before time t3. (When the CO2 gas concentration in the measurement atmosphere is, for example, zero, the heating temperatures of thermistors 11 and 12 reach 150°C and 300°C, respectively.) Therefore, sampling detection signal Vgas at time t2 allows accurate gas concentration measurement.

[0083] However, when gas flow occurs in the measurement atmosphere, thermistors 11 and 12 are cooled by the gas flow. Therefore, at time t2, the heating temperatures of thermistors 11 and 12 have not yet reached 150°C and 300°C, respectively. Based on flow rate signal S, control circuit 35 extends the application time of heater voltages V13 and V14 to a period T2 (>T1) between time t1 and time t5 to compensate for this shortfall in heating time. This ensures that thermistors 11 and 12 are accurately heated to approximately 150°C and 300°C, respectively, even when gas flow occurs in the measurement atmosphere. Furthermore, sampling detection signal Vgas at time t4, before time t5, allows accurate gas concentration measurement.

[0084] In this manner, the gas sensor 300 of the third embodiment changes the heating time of heaters 13 and 14 by varying the application time of heater voltages V13 and V14 based on flow velocity signal S. The application time of heater voltages V13 and V14 is determined by flow velocity signal S. The higher the flow velocity indicated by flow velocity signal S, the longer heater voltages V13 and V14 are applied. This allows accurate CO2 gas concentration detection regardless of the gas flow velocity in the measurement atmosphere.

[0085] While the embodiments of the technology disclosed herein have been described above, the technology disclosed herein is not limited to the above embodiments, and various modifications can be made without departing from the spirit and scope thereof, and such modifications are naturally included within the scope of the technology disclosed herein.

[0086] For example, in the above embodiments, a thermistor, which is a resistor, is used as the temperature sensing element of the sensor unit 10, but the present invention is not limited thereto. For example, a resistor, which is platinum (Pt) or tungsten (W), may be used as the temperature sensing element.

[0087] In addition, in the second embodiment, the levels of the heater voltages V13 and V14 are changed according to the flow rate signal S, and in the third embodiment, the application time of the heater voltages V13 and V14 is changed according to the flow rate signal S, but both the level and application time of the heater voltages V13 and V14 can also be changed according to the flow rate signal S.

[0088] The technology disclosed herein includes the following configuration examples, but is not limited thereto.

[0089] A gas sensor according to one aspect of the present disclosure includes: a sensor unit that generates a detection signal corresponding to the concentration of a target gas; and a control circuit that calculates an output signal representing the concentration of the target gas based on the detection signal. The sensor unit includes a temperature sensing element and a heater that heats the temperature sensing element. The control circuit corrects the output signal or changes the heating conditions of the heater based on a flow rate signal representing the flow rate of gas in a measurement atmosphere. This allows accurate gas concentration detection regardless of the flow rate of gas in the measurement atmosphere.

[0090] In the above-mentioned gas sensor, the control circuit may include a concentration correction table, and the output signal may be corrected by referring to the concentration correction table based on the flow rate signal.

[0091] The gas sensor may further include a differential amplifier for amplifying the potential difference between the detection signal and the reference signal, and the control circuit may change the level of the reference signal according to the flow rate signal.

[0092] In the above-mentioned gas sensor, the control circuit may change the power applied to the heater according to the flow rate signal. This allows the temperature sensing element to be heated to a desired temperature even when a gas flow occurs in the measurement atmosphere.

[0093] In the above-mentioned gas sensor, the control circuit may change the heating time of the heater according to the flow rate signal. This allows the temperature sensing element to be heated to a desired temperature even when a gas flow occurs in the measurement atmosphere.

[0094] Alternatively, the gas sensor may further include a temperature sensor that generates a temperature signal according to the ambient temperature, and the control circuit may vary the power applied to the heater according to the temperature signal.

Claims

1. A gas sensor, wherein: have: a sensor unit that generates a detection signal corresponding to the concentration of the detection target gas; and a control circuit that calculates an output signal indicating the concentration of the detection target gas based on the detection signal, The sensor unit includes a temperature sensing element and a heater for heating the temperature sensing element. The control circuit corrects the output signal or changes the heating condition of the heater based on the flow rate signal indicating the flow rate of the gas in the measurement atmosphere.

2. The gas sensor according to claim 1, wherein The control circuit includes a concentration correction table, and corrects the output signal by referring to the concentration correction table based on the flow rate signal.

3. The gas sensor according to claim 1, wherein The gas sensor further includes: a differential amplifier that amplifies the potential difference between the detection signal and a reference signal; The control circuit changes the level of the reference signal according to the flow rate signal.

4. The gas sensor according to claim 1, wherein The control circuit changes power applied to the heater according to the flow rate signal.

5. The gas sensor according to claim 1, wherein The control circuit changes the heating time of the heater according to the flow rate signal.

6. The gas sensor according to any one of claims 1 to 5, wherein The gas sensor further comprises: a temperature sensor which generates a temperature signal according to the ambient temperature; The control circuit changes power applied to the heater according to the temperature signal.

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