Gas sensor

Abstract

To provide a gas sensor capable of accurately measuring gas concentration regardless of a gas flow rate in a measurement atmosphere.SOLUTION: A gas sensor 100 includes a sensor part 10 for generating a detection signal Vgas according to the concentration of gas to be detected and a control circuit 35 for calculating an output signal Vout showing the concentration of the gas to be detected on the basis of the detection signal Vgas. The sensor part 10 includes thermistors 11 and 12 and heaters 13 and 14 for heating the thermistors 11 and 12; the control circuit 35 corrects the output signal Vout according to a flow rate signal S showing a gas flow rate in a measurement atmosphere; and gas sensors 200 and 300 change the heating conditions of the heaters 13 and 14 according to the flow rate signal S.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Patent Document 1 discloses a gas sensor that can reduce measurement errors caused by gases other than the gas to be detected. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 7070175 Summary of the Invention [Problem to be solved by the invention]

[0004] The present inventors have found that measurement errors occur depending on the gas flow rate in the measurement atmosphere.

[0005] The present disclosure describes a technology relating to a gas sensor that can accurately measure gas concentrations regardless of the gas flow rate in the measurement atmosphere. [Means for solving the problem]

[0006] A gas sensor according to one aspect of the present disclosure comprises a sensor unit that generates a detection signal corresponding to the concentration of a target gas to be detected, and a control circuit that calculates an output signal indicating the concentration of the target gas based on the detection signal, wherein the sensor unit includes a temperature-sensing element and a heater that heats the temperature-sensing element, and the control circuit corrects the output signal or changes the heating conditions of the heater according to a flow rate signal that indicates the flow rate of the gas in the measurement atmosphere. [Effects of the Invention]

[0007] According to the present disclosure, a gas sensor capable of accurately measuring gas concentration regardless of the gas flow rate in the measurement atmosphere is provided. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a circuit diagram showing a configuration of a gas sensor 100 according to a first embodiment of the technique disclosed herein. [Figure 2] FIG. 2 shows an example of the reference voltage generating circuit 32. As shown in FIG. [Figure 3] FIG. 3 is a graph for explaining the influence of the gas flow rate in the measurement atmosphere on the measurement results. [Figure 4] FIG. 4 is a circuit diagram showing a configuration of a gas sensor 200 according to a second embodiment of the technique disclosed herein. [Figure 5] FIG. 5 is a timing chart for explaining a method of correcting heater voltages V13 and V14 using heater voltage correction table 35b. [Figure 6] FIG. 6 is a circuit diagram showing a configuration of a gas sensor 300 according to a third embodiment of the technique disclosed herein. [Figure 7] FIG. 7 is a timing chart for explaining a method of correcting the heating time using the heating time correction table 35c. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the technology according to the present disclosure will be described in detail with reference to the accompanying drawings.

[0010] First Embodiment FIG. 1 is a circuit diagram showing a configuration of a gas sensor 100 according to a first embodiment of the technique disclosed herein.

[0011] 1, the gas sensor 100 according to the first embodiment includes a sensor unit 10 that generates a detection signal Vgas corresponding to the concentration of a gas to be detected, 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 according to this embodiment is a thermal conduction type gas sensor for detecting the concentration of CO2 gas in a measurement atmosphere.

[0012] The sensor unit 10 includes thermistors 11 and 12 connected in series between a power supply Vcc and a ground GND, and heaters 13 and 14 for heating the thermistors 11 and 12, respectively. The detection signal Vgas output from the sensor unit 10 appears at a connection point N1 between the thermistors 11 and 12. The thermistor 11 is a temperature-sensing element for detection, and the thermistor 12 is a temperature-sensing element for reference. The thermistors 11 and 12 are resistors whose resistance changes with temperature. Examples of materials for the thermistors 11 and 12 and the thermistor 22 (described later) include vanadium oxide, amorphous silicon, polycrystalline silicon, oxides with a spinel crystal structure containing manganese, titanium oxide, and yttrium-barium-copper oxide.

[0013] When the thermistor 11, a temperature-sensing element used for detection, is heated to around 150°C and CO2 gas is present in the measurement atmosphere, the heat dissipation characteristics of the thermistor 11 change depending on the concentration of CO2 gas. This change manifests itself as a change in the temperature of thermistor 11, i.e., a change in the resistance value of thermistor 11. Specifically, because CO2 gas has lower heat dissipation properties than air, the temperature of thermistor 11 increases as the CO2 gas concentration increases. Therefore, if the thermistor 11 is heated to 150°C when the CO2 gas concentration in the measurement atmosphere is zero, for example, and CO2 gas is present in the measurement atmosphere, the temperature of thermistor 11 will exceed 150°C depending on the concentration of CO2 gas. As a result, the resistance value of thermistor 11 decreases as the CO2 gas concentration in the measurement atmosphere increases.

[0014] On the other hand, even if CO2 gas is present in the measurement atmosphere when the reference temperature-sensing element 12 is heated to around 300°C, the heat dissipation characteristics of the thermistor 12 change very little depending on the CO2 gas concentration, and the temperature of the thermistor 12 also changes very little. Therefore, the change in resistance value of the thermistor 12 heated to around 300°C due to the CO2 gas concentration is much smaller than the change in resistance value of the thermistor 11 heated to around 150°C due to the CO2 gas concentration. It is not necessary for the thermistor 12 heated to around 300°C to show almost no change in resistance value due to the CO2 gas concentration. As a result, when the thermistor 11 is heated to approximately 150°C and the thermistor 12 is heated to approximately 300°C (for example, when the CO2 gas concentration in the measurement atmosphere is zero, the thermistor 11 is heated to 150°C and the 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 the thermistors 11 and 12. On the other hand, even if the measurement atmosphere contains another gas whose heat dissipation characteristics are not significantly different between when the thermistor 11 is heated to approximately 150°C and when the thermistor 12 is heated to approximately 300°C, the concentration of that gas has almost no effect on the detection signal Vgas. This enables the sensor unit 10 to selectively detect the concentration of CO2 gas.

[0015] 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 thermistor 22. The temperature sensor 20 detects the ambient temperature, which is the temperature of the atmosphere being measured. The temperature sensor 20 may be designed so as not to be affected by heating by the heaters 13 and 14, for example, or so as to be less affected by it.

[0016] The signal processing circuit 30 includes a multiplexer 31, a reference voltage generating circuit 32, a differential amplifier 33, an AD converter (ADC) , a control circuit 35, and a drive circuit .

[0017] 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 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 may be composed of a DA converter 32a that performs DA conversion of the digital value output from the control circuit 35, as shown in FIG. 2(a), or may be composed of variable resistors VR1 and VR2 whose resistance values ​​are controlled by the control circuit 35, as shown in FIG. 2(b).

[0018] The amplified signal Vamp output from the differential amplifier 33 is input to the AD converter 34. The AD converter 34 performs AD conversion on the amplified signal Vamp to generate a digital value, and supplies this to the control circuit 35.

[0019] The control circuit 35 calculates the concentration of CO2 gas, which is the gas to be detected, 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. A calculation formula set in the control circuit 35 is used to calculate the concentration of CO2 gas. Furthermore, the control circuit 35 controls the levels of heater voltages V13 and V14 supplied to the heaters 13 and 14, respectively, via the drive circuit 36.

[0020] The control circuit 35 corrects the heater voltages V13 and V14 in accordance with the amplified signal Vamp obtained by amplifying the 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, the control circuit 35 corrects the heater voltages V13 and V14 so that the temperatures of the thermistors 11 and 12 reach 150°C and 300°C, respectively, after heating by the heaters 13 and 14 for a predetermined time, regardless of the ambient temperature. In other words, the control circuit 35 changes the levels of the heater voltages V13 and V14 in accordance with the temperature signal Vtemp (amplified signal Vamp) to change the power applied to the heaters 13 and 14, thereby changing the heat generation amounts of the heaters 13 and 14.

[0021] Furthermore, the control circuit 35 corrects the output signal Vout in accordance with the flow velocity signal S supplied from the flow velocity sensor 40. The flow velocity sensor 40 may be a part of the gas sensor 100, or may be a device external to the gas sensor 100. The output signal Vout is corrected by referring to a concentration correction table 35a set in the control circuit 35. The concentration correction table 35a is a data table showing the relationship between the flow velocity signal S and the correction amount required for the output signal Vout. The flow velocity signal S is a signal that indicates the flow velocity of the gas in the measurement atmosphere. The higher the flow velocity signal S indicates, the lower the heating temperature of the thermistors 11 and 12 becomes, so the control circuit 35 corrects this.

[0022] FIG. 3 is a graph for explaining the influence of the gas flow rate in the measurement atmosphere on the measurement results, showing the relationship between the heating temperature of thermistors 11 and 12 and the detection sensitivity.

[0023] 3, the detection sensitivity of thermistors 11 and 12, i.e., the relationship between the CO2 gas concentration in the measurement atmosphere and the resistance value of thermistors 11 and 12, varies significantly depending on the heating temperature of thermistors 11 and 12. That is, the CO2 gas detection sensitivity of thermistors 11 and 12 is maximum at approximately 150°C, but is nearly zero in the temperature range of 300°C or higher. Therefore, as described above, by heating thermistor 11 to approximately 150°C and thermistor 12 to approximately 300°C, it is possible to selectively detect the concentration of CO2 gas.

[0024] However, if a gas flow occurs in the measurement atmosphere, the thermistors 11 and 12 are cooled by the gas flow, and the heating temperature of the thermistors 11 and 12 decreases. Therefore, as the flow velocity increases, the heating temperature of thermistor 11 decreases as shown by arrow A in FIG. 3. The heating temperature falls below 150°C, and the detection sensitivity decreases. This reduces the amount of change in the detection signal Vgas relative to changes in CO2 gas concentration. Furthermore, as the flow velocity increases, the heating temperature of thermistor 12 decreases as shown by arrow B in FIG. 3. The heating temperature falls below 300°C, and the detection sensitivity increases. This reduces the resistance of thermistor 12 depending on the CO2 gas concentration, and thus reduces the amount of change in the detection signal Vgas relative to changes in CO2 gas concentration. Thus, if a gas flow occurs in the measurement atmosphere, the detection signal Vgas changes depending on the flow velocity.

[0025] The control circuit 35 corrects the output signal Vout using the concentration correction table 35a so that measurement errors due to such flow velocity are canceled. The amount of correction for the output signal Vout is determined by the flow velocity signal S, and the greater the flow velocity indicated by the flow velocity signal S, the greater the correction for the output signal Vout. This makes it possible to accurately detect the CO2 gas concentration regardless of the flow velocity of the measurement atmosphere.

[0026] Furthermore, the control circuit 35 may change the level of the reference signal Vref in accordance with the flow velocity signal S. This allows for canceling out any offset that may occur in the midpoint level of the detection signal Vgas (the level of the detection signal Vgas that appears at the connection point N1 when the CO2 gas concentration in the measurement atmosphere is, for example, zero) due to the flow velocity of the gas in the measurement atmosphere, thereby preventing a reduction in the dynamic range.

[0027] As described above, the gas sensor 100 according to the first embodiment corrects the output signal Vout in accordance with the flow velocity signal S, and therefore is able to accurately detect the concentration of CO2 gas regardless of the gas flow velocity in the measurement atmosphere.

[0028] Measurement errors due to the flow velocity of gas in the measurement atmosphere can also be canceled by changing the heating conditions of the heaters 13 and 14. Below, an embodiment in which measurement errors due to the flow velocity are canceled by changing the heating conditions of the heaters 13 and 14 will be described.

[0029] <Second embodiment> FIG. 4 is a circuit diagram showing a configuration of a gas sensor 200 according to a second embodiment of the technique disclosed herein.

[0030] 4, the gas sensor 200 according to the second embodiment differs from the gas sensor 100 according to the first embodiment in that a heater voltage correction table 35b, instead of the concentration correction table 35a, is included in the control circuit 35. Since the other basic configurations are the same as those of the gas sensor 100 according to the first embodiment, the same elements are denoted by the same reference numerals and redundant explanations will be omitted.

[0031] The control circuit 35 corrects the levels of the heater voltages V13 and V14 using a heater voltage correction table 35b so as to cancel measurement errors caused by the gas flow velocity in the measurement atmosphere. The heater voltage correction table 35b is a data table showing the relationship between the flow velocity signal S and the correction amounts of the heater voltages V13 and V14.

[0032] FIG. 5 is a timing chart for explaining a method of correcting heater voltages V13 and V14 using heater voltage correction table 35b.

[0033] 5, heater voltages V13 and V14 are applied to heaters 13 and 14, respectively, during period T1 between time t1 and time t3. Level V13a shown in Fig. 5 is the level of heater voltage V13 when the gas flow rate in the measurement atmosphere is zero, and level V14a shown in Fig. 5 is the level of heater voltage V14 when the gas flow rate in the measurement atmosphere is zero. In other words, when the gas flow rate in the measurement atmosphere is zero, setting the levels of heater voltages V13 and V14 to V13a and V14a, respectively, results in heating temperatures of thermistors 11 and 12 of approximately 150°C and approximately 300°C, respectively (when the CO2 gas concentration in the measurement atmosphere is zero, for example, the heating temperatures of thermistors 11 and 12 are 150°C and 300°C, respectively).

[0034] However, if a gas flow is present 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. To cancel this drop in heating temperature, control circuit 35 sets the levels of heater voltages V13 and V14 to V13b (>V13a) and V14b (>V14a), respectively, in accordance with flow velocity signal S. The corrected level V13b of heater voltage V13 is obtained by adding a correction amount corresponding to flow velocity signal S from heater voltage correction table 35b to level V13a, and the corrected level V14b of heater voltage V14 is obtained by adding a correction amount corresponding to flow velocity signal S from heater voltage correction table 35b to level V14a. As a result, even if a gas flow occurs in the measurement atmosphere, the thermistors 11 and 12 are correctly heated to approximately 150° C. and approximately 300° C., respectively. Then, by sampling the detection signal Vgas at time t2, which is immediately before time t3, it becomes possible to accurately measure the gas concentration.

[0035] As described above, the gas sensor 200 according to the second embodiment changes the power applied to the heaters 13 and 14 by changing the levels of the heater voltages V13 and V14 in response to the flow velocity signal S, thereby changing the heat generation amounts of the heaters 13 and 14. The correction amount for the heater voltages V13 and V14 is determined by the flow velocity signal S, and the greater the flow velocity indicated by the flow velocity signal S, the greater the correction for the levels of the heater voltages V13 and V14. This enables accurate detection of the CO2 gas concentration regardless of the flow velocity of the measurement atmosphere.

[0036] <Third embodiment> FIG. 6 is a circuit diagram showing a configuration of a gas sensor 300 according to a third embodiment of the technique disclosed herein.

[0037] 6, the gas sensor 300 according to the third embodiment differs from the gas sensor 100 according to the first embodiment in that a heating time correction table 35c, instead of the concentration correction table 35a, is included in the control circuit 35. Since the other basic configurations are the same as those of the gas sensor 100 according to the first embodiment, the same elements are denoted by the same reference numerals and redundant explanations will be omitted.

[0038] The control circuit 35 corrects the application times of the heater voltages V13 and V14 using a heating time correction table 35c so as to cancel measurement errors caused by the gas flow velocity in the measurement atmosphere. The heating time correction table 35c is a data table showing the relationship between the flow velocity signal S and the application times of the heater voltages V13 and V14.

[0039] FIG. 7 is a timing chart for explaining a method of correcting the heating time using the heating time correction table 35c.

[0040] 7, when the gas flow rate in the measurement atmosphere is zero, heater voltages V13 and V14 are applied to heaters 13 and 14, respectively, during period T1 between time t1 and time t3. When the gas flow rate 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, just before time t3 (when the CO2 gas concentration in the measurement atmosphere is zero, for example, the heating temperatures of thermistors 11 and 12 are 150°C and 300°C, respectively). Therefore, by sampling detection signal Vgas at time t2, the gas concentration can be accurately measured.

[0041] However, if a gas flow occurs in the measurement atmosphere, the thermistors 11 and 12 are cooled by the gas flow, and the heating temperatures of thermistors 11 and 12 do not reach 150°C and 300°C, respectively, even at time t2. To compensate for this lack of heating time, the control circuit 35 extends the application time of heater voltages V13 and V14 to period T2 (>T1) between time t1 and time t5 in response to the flow rate signal S. This ensures that the thermistors 11 and 12 are properly heated to approximately 150°C and approximately 300°C, respectively, even if a gas flow occurs in the measurement atmosphere. Then, by sampling the detection signal Vgas at time t4, just before time t5, accurate gas concentration measurement becomes possible.

[0042] As described above, the gas sensor 300 according to the third embodiment changes the heating time of the heaters 13 and 14 by changing the application time of the heater voltages V13 and V14 in response to the flow rate signal S. The application time of the heater voltages V13 and V14 is determined by the flow rate signal S, and the heater voltages V13 and V14 are applied for a longer period of time as the flow rate signal S indicates a higher flow rate. This enables accurate detection of the CO2 gas concentration regardless of the gas flow rate in the measurement atmosphere.

[0043] The above describes embodiments of the technology according to the present disclosure, but the technology according to the present disclosure is not limited to the above embodiments, and various modifications are possible within the scope of the gist of the technology, and it goes without saying that these modifications are also included within the scope of the technology according to the present disclosure.

[0044] For example, in each of the above-described 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 to this. For example, platinum (Pt) or tungsten (W), which is a resistor, may be used as the temperature-sensing element.

[0045] In addition, in the second embodiment, the levels of the heater voltages V13 and V14 are changed in response to the flow velocity signal S, and in the third embodiment, the application time of the heater voltages V13 and V14 is changed in response to the flow velocity signal S, but it is also possible to change both the levels and application time of the heater voltages V13 and V14 in response to the flow velocity signal S.

[0046] The technology according to the present disclosure includes, but is not limited to, the following configuration examples.

[0047] 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 indicating the concentration of the target gas based on the detection signal, the sensor unit including a temperature-sensing element and a heater that heats the temperature-sensing element, and the control circuit corrects the output signal or changes the heating conditions of the heater in response to a flow rate signal that indicates the flow rate of the gas in a measurement atmosphere. This makes it possible to accurately detect the gas concentration regardless of the flow rate of the gas in the measurement atmosphere.

[0048] In the above gas sensor, the control circuit may have a concentration correction table and may correct the output signal by referring to the concentration correction table in accordance with the flow velocity signal, thereby enabling the output signal to be correctly corrected in accordance with the flow velocity of the gas in the measurement atmosphere.

[0049] The gas sensor may further include a differential amplifier that amplifies the potential difference between the detection signal and the reference signal, and the control circuit may change the level of the reference signal in response to the flow velocity signal, thereby ensuring a sufficient dynamic range.

[0050] In the gas sensor described above, the control circuit may vary the power applied to the heater in response to the flow rate signal, thereby making it possible to heat the temperature sensing element to a desired temperature even when a gas flow is occurring in the measurement atmosphere.

[0051] In the gas sensor described above, the control circuit may vary the heating time of the heater in response to the flow rate signal, thereby making it possible to heat the temperature sensor to a desired temperature even when a gas flow is occurring in the measurement atmosphere.

[0052] The gas sensor may further include a temperature sensor that generates a temperature signal in response to the ambient temperature, and the control circuit may vary the power applied to the heater in response to the temperature signal, thereby enabling the temperature sensor to heat the thermosensitive element to a desired temperature regardless of the ambient temperature. [Explanation of symbols]

[0053] 10 Sensor section 11,12 Thermistor 13,14 Heater 20 Temperature Sensor 21 Resistance 22 Thermistor 30 Signal processing circuit 31 Multiplexer 32 Reference voltage generation circuit 32a DA converter 33 Differential Amplifier 34 AD converter 35 Control circuit 35a Density correction table 35b Heater voltage correction table 35c Heating time correction table 36 Drive circuit 40 Flow velocity sensor 100, 200, 300 Gas Sensor N1,N2 connection points S flow rate signal VR1, VR2 variable resistors Vamp Amplified signal Vgas detection signal Vout output signal Vref reference signal Vtemp temperature signal

Claims

1. a sensor unit that generates a detection signal corresponding to the concentration of the target gas; a control circuit that calculates an output signal indicating the concentration of the target gas based on the detection signal; Equipped with 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 in response to a flow velocity signal indicating the flow velocity of the gas in the measurement atmosphere. Gas sensor.

2. the control circuit has a concentration correction table, and corrects the output signal by referring to the concentration correction table in accordance with the flow velocity signal; 2. The gas sensor according to claim 1.

3. a differential amplifier that amplifies a potential difference between the detection signal and a reference signal; the control circuit changes the level of the reference signal in response to the flow velocity signal; 2. The gas sensor according to claim 1.

4. the control circuit varies the power applied to the heater in response to the flow rate signal; 2. The gas sensor according to claim 1.

5. The control circuit changes the heating time of the heater in response to the flow rate signal.

2. The gas sensor according to claim 1.

6. a temperature sensor that generates a temperature signal in response to an environmental temperature; the control circuit changes the power applied to the heater in response to the temperature signal; The gas sensor according to claim 1 .

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