Gas sensor

Abstract

[Problem] To expand a dynamic range while enhancing resolution in a gas sensor equipped with an amplifier that amplifies a detection voltage according to the concentration of a detection target gas. [Solution] A gas sensor 100 comprises: a sensor unit 10 in which a detection voltage Vgas changes according to the concentration of a detection target gas; an amplifier 31 that amplifies the detection voltage Vgas; and a control circuit 37 that calculates the concentration of the detection target gas on the basis of an output voltage Vamp 1 from the amplifier 31. The control circuit 37 changes amplification factors of the amplifier 31 according to the concentration of the detection target gas and also changes calculation formulas for use in calculating the concentration of the detection target gas on the basis of the output voltage Vamp 1.

Description

Gas Sensor

[0001] The present disclosure relates to a gas sensor, and more particularly to a gas sensor including an amplifier that amplifies a detection voltage in accordance with the concentration of a gas to be detected.

[0002] Japanese Patent Application Laid-Open No. 2003-144992 discloses a gas sensor equipped with an amplifier that amplifies a detection voltage in accordance with the concentration of a gas to be detected.

[0003] JP 2019-060848 A

[0004] In the gas sensor described in Patent Document 1, the dynamic range is determined by the sensitivity of the sensor unit, the amplification factor of the amplifier, and the power supply voltage. Therefore, if the amplification factor of the amplifier is set to be large in order to improve the resolution, the dynamic range will be narrowed accordingly.

[0005] The present disclosure describes a technology for increasing the resolution and widening the dynamic range in a gas sensor having an amplifier that amplifies a detection voltage according to the concentration of a target gas.

[0006] A gas sensor according to one aspect of the present disclosure comprises a sensor section whose detection voltage changes depending on the concentration of a target gas, an amplifier that amplifies the detection voltage, and a control circuit that calculates the concentration of the target gas based on the output voltage of the amplifier, and the control circuit switches the amplification factor of the amplifier and switches a calculation formula for calculating the concentration of the target gas from the output voltage depending on the concentration of the target gas.

[0007] According to the present disclosure, a technique is provided for increasing the resolution and widening the dynamic range in a gas sensor having an amplifier that amplifies a detection voltage according to the concentration of a gas to be detected.

[0008] FIG. 1 is a circuit diagram showing a configuration of a gas sensor 100 according to an embodiment of the technology disclosed herein. FIG. 2 is a graph illustrating a first example of switching control by the control circuit 37. FIG. 3 is a diagram showing calculation formulas and tables set in the control circuit 37. FIG. 4 is a graph illustrating a second example of switching control by the control circuit 37. FIG. 5 is a graph illustrating a third example of switching control by the control circuit 37. FIG. 6 is a circuit diagram of a circuit that generates a reference voltage Vref using variable resistors VR1 and VR2. FIG. 7 is a graph illustrating a fourth example of switching control by the control circuit 37.

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

[0010] FIG. 1 is a circuit diagram showing a configuration of a gas sensor 100 according to an embodiment of the technology disclosed herein.

[0011] 1, the gas sensor 100 according to this embodiment includes a sensor section 10 whose detection voltage Vgas changes in accordance with the concentration of a gas to be detected, a temperature sensor 20 that generates a temperature signal Vtemp in accordance with the ambient temperature, and a signal processing circuit 30. Although not particularly limited, the gas sensor 100 according to this embodiment detects the CO ２ This is a thermal conduction type gas sensor for detecting gas concentration.

[0012] The sensor unit 10 includes thermistors Rd1 and Rd2 connected in series between a power supply Vcc and ground GND, and heater resistors MH1 and MH2 that heat the thermistors Rd1 and Rd2, respectively. The detection voltage Vgas of the sensor unit 10 appears at a connection point N1 between the thermistors Rd1 and Rd2. The thermistor Rd1 is a detection resistor (a resistor whose resistance value changes with temperature), and thermistor Rd2 is a reference resistor (a resistor whose resistance value changes with temperature). Examples of materials for the thermistors Rd1 and Rd2 and the thermistor Rd3 (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] The thermistor Rd1, which is a resistor for detection, is heated to around 150°C, and CO is introduced into the measurement atmosphere. ２ When CO gas is present, the heat dissipation characteristics of the thermistor Rd1 change depending on the concentration of the gas. This change appears as a change in the temperature of the thermistor Rd1, i.e., a change in the resistance value of the thermistor Rd1. ２ Gas has lower heat dissipation capacity than air, so CO ２ The higher the concentration of the gas, the higher the temperature of the thermistor Rd1. ２ When the gas concentration is zero, for example, if the thermistor Rd1 is heated to a temperature of 150° C., CO ２ If CO gas is present, the temperature of the thermistor Rd1 will rise above 150° C. depending on the concentration of CO gas in the measurement atmosphere. ２ The higher the gas concentration, the lower the resistance value of the thermistor Rd1.

[0014] On the other hand, the thermistor Rd2, which is a reference resistor, is heated to about 300°C, and CO is introduced into the measurement atmosphere. ２ Even if CO gas is present, the heat dissipation characteristics of the thermistor Rd2 hardly change depending on the concentration of the gas, and the temperature of the thermistor Rd2 hardly changes either. ２ The change in resistance due to the gas concentration was measured by the CO ２ The change in resistance due to the gas concentration is much smaller than that due to the CO ２ The resistance value may hardly change depending on the gas concentration. As a result, when the thermistor Rd1 is heated to about 150°C and the thermistor Rd2 is heated to about 300°C (CO in the measurement atmosphere), ２ When the gas concentration is, for example, zero, the thermistor Rd1 is heated to 150° C. and the thermistor Rd2 is heated to 300° C., and the connection point N1 between the thermistors Rd1 and Rd2 is heated to 150° C. and 300° C., respectively. ２The detected voltage Vgas appears according to the concentration of the gas. On the other hand, even if the measurement atmosphere contains another gas whose heat dissipation characteristics when the thermistor Rd1 is heated to around 150°C are not significantly different from those when the thermistor Rd2 is heated to around 300°C, the concentration of that gas has almost no effect on the detected voltage Vgas. As a result, the sensor unit 10 can measure CO ２ It is possible to selectively detect the concentration of a gas.

[0015] 1, a compensation resistor R1 may be connected in parallel to the thermistor Rd2. If there is a difference in the humidity sensitivity between the thermistor Rd1 and the thermistor Rd2, this difference can be cancelled by connecting the compensation resistor R1 in parallel to the thermistor Rd2.

[0016] The temperature sensor 20 includes a thermistor Rd3 and a fixed resistor R2 connected in series between a power supply Vcc and ground GND. The temperature signal Vtemp from the temperature sensor 20 appears at a connection point N2 between the thermistor Rd3 and the fixed resistor R2. The temperature sensor 20 detects the ambient temperature, which is the temperature of the atmosphere being measured. The temperature sensor 20 may be designed to be unaffected or less susceptible to heating by heater resistors MH1 and MH2, for example.

[0017] The signal processing circuit 30 includes differential amplifiers 31 to 33, a buffer 34, an AD converter (ADC) 35, a DA converter (DAC) 36, and a control circuit 37. Like the sensor unit 10, each circuit constituting the signal processing circuit 30 operates on the voltage between the power supply Vcc and the ground GND.

[0018] The differential amplifier 31 compares the detection voltage Vgas with the reference voltage Vref to generate an output voltage Vamp1, which is an amplified difference between the detection voltage Vgas and the reference voltage Vref (=Vgas-Vref). The buffer 34 buffers the temperature signal Vtemp to generate an output voltage Vamp2. The output voltages Vamp1 and Vamp2 are input to an AD converter 35. The AD converter 35 performs AD conversion on the output voltages Vamp1 and Vamp2 to generate digital values, which are then supplied to a control circuit 37.

[0019] The control circuit 37 controls the detection target gas, CO ２ Calculate the gas concentration and CO ２ It generates an output voltage Vout that indicates the concentration of the CO gas. ２ A calculation formula set in the control circuit 37 is used to calculate the gas concentration. Furthermore, the control circuit 37 supplies digital values ​​of various control parameters to the DA converter 36. The DA converter 36 converts the digital values ​​of the various control parameters into analog values ​​to generate heater voltages Vmh1, Vmh2 and a reference voltage Vref. The heater voltage Vmh1 is applied to the heater resistor MH1 via a differential amplifier 32 that forms a voltage follower, thereby heating the thermistor Rd1. The heater voltage Vmh2 is applied to the heater resistor MH2 via a differential amplifier 33 that also forms a voltage follower, thereby heating the thermistor Rd2. Furthermore, the reference voltage Vref is supplied to the differential amplifier 31.

[0020] The control circuit 37 corrects the heater voltages Vmh1 and Vmh2 in response to the temperature signal Vtemp. ２ When the gas concentration is, for example, zero, the heater voltages Vmh1 and Vmh2 are corrected so that the temperatures of the thermistors Rd1 and Rd2 become 150° C. and 300° C., respectively. The control circuit 37 also corrects the reference voltage Vref in response to the temperature signal Vtemp. This is because the CO ２Even if the gas concentration is constant, the correction resistor R1 has slight temperature characteristics, and the temperatures of the thermistors Rd1 and Rd2 cannot be completely controlled by simply correcting the heater voltages Vmh1 and Vmh2. For these reasons, not only does an offset occur in the level of the detection voltage Vgas that appears at the connection point N1 depending on the ambient temperature, but the differential amplifiers 31 to 33 also have slight temperature characteristics, and the reference voltage Vref is corrected to cancel out the effects of these ambient temperatures.

[0021] Furthermore, the control circuit 37 switches the gain of the differential amplifier 31 in accordance with the calculated output voltage Vout. ２ The more the gas concentration is indicated, the lower the amplification factor of the differential amplifier 31 is. ２ In an environment with a low gas concentration, the amplification factor of the differential amplifier 31 is set high, so that high resolution can be obtained. ２ In an environment with a high gas concentration, the amplification factor of the differential amplifier 31 is set low, thereby preventing saturation of the differential amplifier 31. This ensures a wide dynamic range while providing high resolution in a low-concentration environment where highly accurate detection is required.

[0022] FIG. 2 is a graph for explaining a first example of switching control by the control circuit 37, and shows the relationship between CO ２ 1 shows the relationship between the gas concentration and the output voltage Vamp1 of the differential amplifier 31.

[0023] In the example shown in FIG. 2, CO ２ The amplification factor of the differential amplifier 31 is switched at the gas concentration P1. ２ In the concentration range A1 where the gas concentration is less than P1, the amplification factor of the differential amplifier 31 is set to xA1 (for example, 50 times), and the CO ２In a concentration range A2 where the gas concentration is equal to or greater than P1, the amplification factor of the differential amplifier 31 is set to xA2 (<xA1) (e.g., 5x). The dashed line α in FIG. 2 indicates the output voltage Vamp1 when the amplification factor of the differential amplifier 31 is fixed at xA1 (e.g., 50x). It can be seen that the output voltage Vamp1 saturates at Vcc at a certain concentration. In contrast, in the example shown in FIG. 2, the amplification factor of the differential amplifier 31 is reduced to xA2 in the concentration range A2, so that the output voltage Vamp1 does not saturate and measurement is possible up to the concentration reaching 100%. However, it is not necessary to set the amplification factor of the differential amplifier 31 so that the output voltage Vamp1 does not saturate at 100% concentration. It is also possible to design the output voltage Vamp1 to saturate in a concentration range beyond the required measurement range.

[0024] As an example, CO in the measurement atmosphere ２ If the reference voltage Vref is set so that the output voltage Vamp1 of the differential amplifier 31 becomes 0 V when the gas concentration is zero, for example, the sensitivity of the sensor unit 10 becomes 2×10 －７ If V / ppm, concentration P1 is 1%, amplification factor xA1 is 50, and amplification factor xA2 is 5, the output voltage Vamp1 at concentration P1 is 0.1 V, and the output voltage Vamp1 at concentration 100% is 1.09 V. For this reason, if a voltage of, for example, 2.7 V is used as the power supply Vcc, the output voltage Vamp1 will not saturate.

[0025] When the amplification factor of the differential amplifier 31 is switched, ２ The relationship between the gas concentration and the output voltage Vamp1 changes. Therefore, the control circuit 37 changes the output voltage Vamp1 to CO ２ The calculation formula for calculating the concentration of the gas is switched, and thereby, a correct output voltage Vout can be generated. ２ The formula fx for calculating the gas concentration is set, and the current CO ２ When the gas concentration is in the concentration range A1, the calculation formula fx1 is selected, and the current CO ２When the gas concentration is in the concentration range A2, by selecting the calculation formula fx2, ２ As described above, the control circuit 37 corrects the reference voltage Vref in accordance with the temperature signal Vtemp, and the reference voltage Vref is determined by the control circuit 37 by calculating, for example, the following equation:

[0026]

[0027] In the above formula, a, b, c, and d are constants. As shown in FIG. 3, the control circuit 37 has a table relating to the constants a, b, c, and d, and calculates the current CO ２ When the gas concentration is in the concentration range A1, constants a1, b1, c1, and d1 are selected, and the current CO ２ When the gas concentration is in the concentration range A2, the constants a2, b2, c2, and d2 are selected, thereby making it possible to generate an appropriate reference voltage Vref according to the amplification factor of the differential amplifier 31.

[0028] FIG. 4 is a graph for explaining a second example of switching control by the control circuit 37. In FIG.

[0029] In the example shown in FIG. 4, CO ２ The amplification factor of the differential amplifier 31 is switched between three stages, with the gas concentrations P2 and P3 as boundaries. ２ In the concentration range B1 where the gas concentration is less than P2, the amplification factor of the differential amplifier 31 is set to xB1, and the CO ２ In the concentration range B2 where the gas concentration is equal to or greater than P2 and less than P3, the amplification factor of the differential amplifier 31 is set to xB2 (<xB1), and the CO ２ In a concentration range B3 where the gas concentration is equal to or higher than P3, the amplification factor of the differential amplifier 31 is set to xB3 (<xB2), which results in a higher resolution in the concentration range B2 than in the concentration range B3.

[0030] As illustrated in the second example shown in FIG. 4, it is also possible to switch the amplification factor of the differential amplifier 31 in three or more stages.

[0031] FIG. 5 is a graph for explaining a third example of switching control by the control circuit 37. In FIG.

[0032] In the example shown in FIG. 5, CO ２ The amplification factor of the differential amplifier 31 is switched and the reference voltage Vref is also switched at the gas concentration P4. ２ The level of the output voltage Vamp1 relative to the gas concentration (concentration P4) is reset to a reference voltage (for example, 0 V). ２ In the concentration range C1 where the gas concentration is less than P4, the amplification factor of the differential amplifier 31 is set to xC1, and the CO ２ The reference voltage Vref is set so that the output voltage Vamp1 of the differential amplifier 31 becomes, for example, 0 V when the gas concentration is zero. ２ In the concentration range C2 where the gas concentration is equal to or higher than P4, the amplification factor of the differential amplifier 31 is set to xC2 (<xC1), and the CO ２ The reference voltage Vref is set so that when the gas concentration is P4, the output voltage Vamp1 of the differential amplifier 31 is, for example, 0 V. This allows the amplification factor xC2 in the high concentration region to be set higher, thereby improving the resolution in the high concentration region.

[0033] When switching the reference voltage Vref, correction of the reference voltage Vref according to the temperature signal Vtemp (switching the selection of the constants a, b, c, and d in the formula for the reference voltage Vref described above) may be performed at the same time. In this case, in the concentration range C1, the CO ２ When the gas concentration is zero, the reference voltage Vref is set so that the output voltage Vamp1 of the differential amplifier 31 is, for example, 0 V regardless of the environmental temperature. ２ When the gas concentration is P4, the reference voltage Vref is set so that the output voltage Vamp1 of the differential amplifier 31 is, for example, 0 V regardless of the ambient temperature.

[0034] As illustrated in the third example shown in FIG. 5, it is also possible to switch the reference voltage in conjunction with switching the amplification factor of the differential amplifier 31.

[0035] The reference voltage Vref may be switched by changing the digital value of the reference voltage Vref supplied from the control circuit 37 to the DA converter 36. Alternatively, as shown in Fig. 6, when the reference voltage Vref is generated by a series circuit of variable resistors VR1 and VR2, the reference voltage Vref may be switched by switching the resistance values ​​of the variable resistors VR1 and VR2 using the control circuit 37.

[0036] FIG. 7 is a graph for explaining a fourth example of switching control by the control circuit 37. In FIG.

[0037] In the example shown in FIG. 7, CO ２ The amplification factor of the differential amplifier 31 is switched and the reference voltage Vref is also switched at the gas concentration P5. ２ The level of the output voltage Vamp1 relative to the gas concentration (concentration P5) is reset to a reference voltage (for example, 0 V). ２ The reference voltage Vref is switched at the gas concentration P6 as a boundary, thereby ２ The level of the output voltage Vamp1 for the gas concentration (concentration P6) is reset to a reference voltage (for example, 0 V). ２ In the concentration range D1 where the gas concentration is less than P5, the amplification factor of the differential amplifier 31 is set to xD1, and the CO ２ The reference voltage Vref is set so that the output voltage Vamp1 of the differential amplifier 31 becomes, for example, 0 V when the gas concentration is zero. ２ In the concentration range D2a where the gas concentration is equal to or greater than P5 and less than P6, the amplification factor of the differential amplifier 31 is set to xD2 (<xD1), and the CO ２The reference voltage Vref is set so that the output voltage Vamp1 of the differential amplifier 31 becomes, for example, 0 V when the gas concentration is P5. ２ In the concentration range D2b where the gas concentration is equal to or higher than P6, the amplification factor of the differential amplifier 31 is set to xD2 (<xD1), and the CO ２ The reference voltage Vref is set so that the output voltage Vamp1 of the differential amplifier 31 becomes, for example, 0 V when the gas concentration is P6.

[0038] When switching the reference voltage Vref, correction of the reference voltage Vref according to the temperature signal Vtemp (switching the selection of the constants a, b, c, and d in the formula for the reference voltage Vref described above) may also be performed. In this case, in the concentration range D1, the CO ２ When the gas concentration is zero, the reference voltage Vref is set so that the output voltage Vamp1 of the differential amplifier 31 is, for example, 0 V regardless of the environmental temperature. ２ When the gas concentration is P5, the reference voltage Vref is set so that the output voltage Vamp1 of the differential amplifier 31 is, for example, 0 V regardless of the ambient temperature. In the concentration range D2b, the CO ２ When the gas concentration is P6, the reference voltage Vref is set so that the output voltage Vamp1 of the differential amplifier 31 is, for example, 0 V regardless of the ambient temperature.

[0039] 7, it is also possible to switch the reference voltage independently of switching the gain of the differential amplifier 31. Furthermore, as in the fourth example shown in Fig. 7, by switching the reference voltage using a plurality of gas concentrations as boundaries, the gain xD2 in the high-concentration region can be set even higher, thereby further improving the resolution in the high-concentration region.

[0040] 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.

[0041] For example, in the above embodiment, a thermistor, which is a resistor, is used in the sensor unit 10, but the present invention is not limited to this. For example, in a thermal conduction type gas sensor, platinum (Pt) or tungsten (W) may be used as a resistor in the sensor unit. Also, for example, in a catalytic combustion type gas sensor, the sensor unit may be formed by supporting precious metal particles such as platinum (Pt), palladium (Pd), ruthenium (Ru), or rhodium (Rh) on a carrier of an oxide material such as aluminum oxide (gamma alumina, etc.) or silicon oxide. Also, for example, in a semiconductor type gas sensor, the sensor unit may be formed by using a metal oxide film that undergoes an oxidation-reduction reaction upon contact with the gas to be detected. Examples of metal oxides that constitute such a metal oxide film include tin oxide (SnO ２ ), zirconium oxide (ZrO ２ ), iron oxide (Fe ２ O ３ ), tungsten oxide (WO ３ ), indium oxide (In ２ O ３ ) or cobalt oxide (Co ３ O ４ In addition, for example, an NDIR (non-dispersive infrared) gas sensor uses an infrared sensor that detects infrared rays in the sensor portion.

[0042] The technology according to the present disclosure includes, but is not limited to, the following configuration examples.

[0043] A gas sensor according to one aspect of the present disclosure includes a sensor unit whose detection voltage changes in response to the concentration of a target gas, an amplifier that amplifies the detection voltage, and a control circuit that calculates the concentration of the target gas based on the output voltage of the amplifier, and the control circuit switches the amplification factor of the amplifier and switches a formula for calculating the concentration of the target gas from the output voltage in response to the concentration of the target gas, thereby ensuring a wide dynamic range and improving resolution within a predetermined concentration range.

[0044] In the above gas sensor, the amplifier may be a differential amplifier that generates an output voltage by amplifying the difference between the detection voltage and the reference voltage by the above-mentioned gain. In this case, the output voltage at any gas concentration can be made zero by setting the reference voltage, thereby further expanding the dynamic range.

[0045] In the above gas sensor, the control circuit may switch the reference voltage in accordance with the concentration of the gas to be detected. This allows for higher resolution. In this case, the control circuit may switch the reference voltage in conjunction with switching the amplification factor. This facilitates control by the control circuit.

[0046] 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 correct the reference voltage in response to the temperature signal. This allows for more accurate calculation of the gas concentration. In this case, the control circuit may switch the correction formula for correcting the reference voltage in conjunction with switching the amplification factor. This allows for even more accurate calculation of the gas concentration.

[0047] In the gas sensor described above, the sensor unit may include a resistor whose resistance value changes with temperature and a heater that heats the resistor, and the change in detection voltage may be caused by a change in the resistance value of the resistor heated by the heater in response to the concentration of the target gas. Such a thermal conduction type gas sensor is particularly suitable for applying the technology of the present disclosure, since, in principle, the linearity of the relationship between gas concentration and detection voltage is maintained over a wide range of gas concentrations.

[0048] This application claims the benefit of Japanese Patent Application No. 2024-034802, filed on March 7, 2024, the entire disclosure of which is incorporated herein by reference.

[0049] 10 Sensor unit 20 Temperature sensor 30 Signal processing circuit 31 to 33 Differential amplifier 34 Buffer 35 AD converter 36 DA converter 37 Control circuit 100 Gas sensor MH1, MH2 Heater resistor N1, N2 Connection point R1 Correction resistor R2 Fixed resistor Rd1 to Rd3 Thermistor VR1, VR2 Variable resistor Vamp1, Vamp2 Output voltage Vgas Detection voltage Vmh1, Vmh2 Heater voltage Vout Output voltage Vref Reference voltage Vtemp Temperature signal

Claims

1. A gas sensor comprising: a sensor section whose detection voltage changes in accordance with the concentration of a target gas; an amplifier which amplifies the detection voltage; and a control circuit which calculates the concentration of the target gas based on the output voltage of the amplifier, wherein the control circuit switches the amplification factor of the amplifier in accordance with the concentration of the target gas, and switches a calculation formula for calculating the concentration of the target gas from the output voltage.

2. The gas sensor according to claim 1, wherein the amplifier is a differential amplifier that generates the output voltage by amplifying the difference between the detection voltage and a reference voltage by the amplification factor.

3. The gas sensor according to claim 2, wherein the control circuit switches the reference voltage depending on the concentration of the target gas.

4. The gas sensor according to claim 3, wherein the control circuit switches the reference voltage in conjunction with switching of the amplification factor.

5. The gas sensor according to any one of claims 2 to 4, further comprising a temperature sensor that generates a temperature signal in response to an environmental temperature, wherein the control circuit corrects the reference voltage in response to the temperature signal.

6. The gas sensor according to claim 5, wherein the control circuit switches a correction formula for correcting the reference voltage in conjunction with switching of the amplification factor.

7. The gas sensor according to claim 1, wherein the sensor section includes a resistor whose resistance value changes with temperature and a heater that heats the resistor, and the change in the detection voltage occurs when the resistance value of the resistor heated by the heater changes in accordance with the concentration of the target gas.