Gas sensor

Abstract

The invention provides an improved gas sensor capable of eliminating the influence of drift. A gas sensor (100) is provided with: a sensor unit (10) that generates a gas detection signal (Vgas) corresponding to the concentration of a gas to be measured; a differential amplifier (33) that generates a differential signal (Vdiff) by amplifying the difference between the gas detection signal (Vgas) and a reference voltage (Vref); and a control circuit (35) that generates an output signal (Vout) indicating the concentration of the gas to be measured on the basis of the differential signal (Vdiff). When the level of the differential signal is lower than a threshold value (Vth) corresponding to the level of the differential signal for determining that the concentration of the gas to be measured is normal, the control circuit (35) corrects the reference voltage (Vref) such that the level of the differential signal (Vdiff) is equal to or greater than the threshold value (Vth) or closer to the threshold value (Vth).

Description

Technical Field

[0001] This disclosure relates to a gas sensor, and more particularly to a gas sensor capable of eliminating negative drift. Background Technology

[0002] Patent document 1 discloses a type of gas sensor that amplifies the detection signal by comparing the detection signal corresponding to the concentration of the gas to be measured with an offset voltage (reference voltage) using a differential amplifier, and calculates the concentration of the gas to be measured based on the amplified detection signal.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent No. 5563507. Summary of the Invention

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

[0007] However, in the gas sensor described in Patent Document 1, there is a problem of calculation error in the concentration of the gas being measured due to the drift caused by changes over time.

[0008] In this disclosure, an improved gas sensor capable of eliminating the effects of drift is described.

[0009] Technical means for solving technical problems

[0010] One aspect of this disclosure is a gas sensor comprising: a sensor unit that generates a gas detection signal corresponding to the concentration of a gas to be measured; a differential amplifier that generates a differential signal by amplifying the difference between the gas detection signal and a reference voltage; and a control circuit that generates an output signal representing the concentration of the gas to be measured based on the differential signal, wherein, when the level of the differential signal is lower than a threshold, the control circuit corrects the reference voltage to make the level of the differential signal above the threshold, wherein the threshold is equivalent to the level of the differential signal at which the concentration of the gas to be measured is determined to be at a normal concentration.

[0011] Another aspect of the gas sensor disclosed herein includes: a sensor unit that generates a gas detection signal corresponding to the concentration of the gas to be measured; a differential amplifier that generates a differential signal by amplifying the difference between the gas detection signal and a reference voltage; and a control circuit that generates an output signal representing the concentration of the gas to be measured based on the differential signal, wherein when the level of the differential signal is lower than a threshold, the control circuit corrects the reference voltage to bring the level of the differential signal closer to the threshold, wherein the threshold is equivalent to the level of the differential signal when the concentration of the gas to be measured is determined to be at its normal concentration.

[0012] The effects of the invention

[0013] According to this disclosure, a gas sensor capable of eliminating negative drift can be provided. Attached Figure Description

[0014] Figure 1 This is a circuit diagram illustrating the structure of a gas sensor 100 according to one embodiment of the technology disclosed herein.

[0015] Figure 2 This is a circuit example of the reference voltage generation circuit 32.

[0016] Figure 3 This is a flowchart illustrating the ambient temperature measurement operation of the gas sensor 100.

[0017] Figure 4 This is a flowchart illustrating the first example of the gas concentration measurement operation of the gas sensor 100.

[0018] Figure 5 This is a flowchart illustrating a modified example of the first example of the gas concentration measurement operation of the gas sensor 100.

[0019] Figure 6 This is a flowchart illustrating the second example of the gas concentration measurement operation of the gas sensor 100.

[0020] Figure 7 This is a schematic graph used to illustrate the first effect of the gas sensor 100.

[0021] Figure 8 This is a schematic graph used to illustrate the second effect of the gas sensor 100.

[0022] Figure 9 This is a schematic graph illustrating an example of the operation of a gas sensor 100 when measuring changes in the concentration of CO2 gas in an atmosphere.

[0023] Figure 10 This is a circuit diagram showing the structure of the gas sensor 100a in the first modified example.

[0024] Figure 11 This is a circuit diagram showing the structure of the gas sensor 100b in the second modified example.

[0025] Figure 12 This is a circuit diagram showing the structure of the gas sensor 100c in the third modified example.

[0026] Explanation of symbols:

[0027] 10. Sensor Department

[0028] 11, 12 Thermistors

[0029] Heaters 13 and 14

[0030] 15 Fixed resistors

[0031] 20 Temperature Sensors

[0032] 21 Thermistor

[0033] 22 Resistors

[0034] 30 Signal Processing Circuit

[0035] 31 Multiplexer

[0036] 32 Reference Voltage Generation Circuit

[0037] 32a DA converter

[0038] 33 Differential Amplifier

[0039] 34 AD converter

[0040] 35 Control Circuit

[0041] 35a~35d memory

[0042] 36. Power supply circuit for sensor components

[0043] 37 Heater drive circuit

[0044] 100, 100a~100c Gas Sensors

[0045] N1, N2 connection points

[0046] VL power wiring

[0047] VR1, VR2 Variable Resistors Detailed Implementation

[0048] In the following text, please refer to the appendix. Figure 1 The embodiments of the technology disclosed herein will be described in detail below.

[0049] Figure 1 This is a circuit diagram illustrating the structure of a gas sensor 100 according to one embodiment of the technology disclosed herein.

[0050] like Figure 1As shown, the gas sensor 100 of this embodiment includes: a sensor unit 10 that generates a gas detection signal Vgas corresponding to the concentration of the gas to be measured; 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 conductivity type gas sensor for detecting the concentration of CO2 gas in a measurement atmosphere.

[0051] The sensor unit 10 includes: thermistors 11 and 12 connected in series between the power supply line VL and ground GND; and heaters 13 and 14 for heating the thermistors 11 and 12 respectively. The gas detection signal Vgas output from the sensor unit 10 appears at the connection point N1 of 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 resistive elements whose resistance changes with temperature. Examples of materials for thermistors 11, 12, and thermistor 21 (described later) include vanadium oxide, amorphous silicon, polycrystalline silicon, manganese-containing spinel-type crystal oxides, titanium oxide, or yttrium-barium-copper oxide.

[0052] When the thermistor 11, used as a temperature sensing element for detection, is heated to a temperature range of 100°C to 230°C, for example, around 150°C, which is highly sensitive to CO2 gas, the heat dissipation characteristics of the thermistor 11 will change depending on the concentration of CO2 gas in the measuring atmosphere. This change manifests as a change in the temperature of the thermistor 11, i.e., a change in the resistance value of the thermistor 11. Specifically, since CO2 gas has lower heat dissipation than air, the higher the concentration of CO2 gas, the higher the temperature of the thermistor 11. Therefore, if the thermistor 11 is heated to 150°C when the CO2 gas concentration in the measuring atmosphere is the same as that in the atmosphere under normal conditions (e.g., 400 ppm), then when the concentration of CO2 gas in the measuring atmosphere exceeds that of the atmosphere under normal conditions, the temperature of the thermistor 11 will be correspondingly higher than 150°C. As a result, the higher the concentration of CO2 gas in the measuring atmosphere, the lower the resistance value of the thermistor 11.

[0053] On the other hand, when the thermistor 12, used as a reference temperature sensing element, is heated to a temperature range of 250°C to 450°C, such as around 300°C, where its sensitivity to CO2 gas is low, the heat dissipation characteristics of the thermistor 12 hardly change with its concentration, even if CO2 gas is present in the measuring atmosphere, and the temperature of the thermistor 12 also hardly changes. Therefore, the change in resistance value of the thermistor 12 heated to around 300°C due to the concentration of CO2 gas is sufficiently smaller than the change in resistance value of the thermistor 11 heated to around 150°C due to the concentration of CO2 gas. The resistance value of the thermistor 12 heated to around 300°C can also remain almost unchanged due to the concentration of CO2 gas. As a result, if thermistor 11 is heated to around 150°C and thermistor 12 is heated to around 300°C (i.e., if the CO2 gas concentration in the measuring atmosphere is the same as the normal atmospheric CO2 gas concentration, and thermistor 11 is heated to 150°C and thermistor 12 to 300°C), a gas detection signal Vgas corresponding to the CO2 gas concentration in the measuring atmosphere appears at the junction N1 of thermistors 11 and 12. On the other hand, even if the measuring atmosphere contains other gases whose heat dissipation characteristics are not significantly different when thermistor 11 is heated to around 150°C compared to when thermistor 12 is heated to around 300°C, the concentration of these gases has almost no effect on the gas detection signal Vgas. Therefore, the sensor unit 10 can selectively detect the CO2 gas concentration.

[0054] Temperature sensor 20 includes a thermistor 21 and a resistor 22 connected in series between the power supply line VL and ground GND. The temperature signal Vtemp of temperature sensor 20 appears at the junction N2 of the thermistor 21 and the resistor 22. Temperature sensor 20 detects the ambient temperature. The ambient temperature is the temperature of the measured atmosphere. Temperature sensor 20 can also be designed to be unaffected or minimally affected by heating from heaters 13 and 14.

[0055] The signal processing circuit 30 includes a multiplexer 31, a reference voltage generation circuit 32, a differential amplifier 33, an AD converter (ADC) 34, a control circuit 35, a sensor element power supply circuit 36, and a heater drive circuit 37.

[0056] The multiplexer 31, under the control of the control circuit 35, supplies one of the gas detection signal Vgas and the temperature signal Vtemp to the differential amplifier 33. The differential amplifier 33 generates a differential signal Vdiff, which is obtained by amplifying the difference (potential difference) between the level of one of the gas detection signal Vgas and the temperature signal Vtemp and the level of the reference voltage Vref generated by the reference voltage generation circuit 32. The differential amplifier 33 can also generate a differential signal Vdiff by amplifying the difference (potential difference) between the level of one of the gas detection signal Vgas and the temperature signal Vtemp and the level of the reference voltage Vref at any amplification factor greater than or less than 1. Figure 2 As shown in (a), the reference voltage generation circuit 32 can be composed of a DA converter (DAC) 32a that performs DA conversion on the digital value output from the control circuit 35, or it can be as follows: Figure 2 As shown in (b), the circuit consists of variable resistors VR1 and VR2 whose resistance values ​​are controlled by control circuit 35. In either case, the level of the reference voltage Vref is determined by the initial value INI stored in memory 35a, the set value REFG stored in memory 35b, or the set value REFT stored in memory 35c, all contained in control circuit 35. The initial value INI can be a fixed value or a switchable value. Furthermore, as described later, the set value REFG is updated during gas concentration measurement.

[0057] There is no particular limitation on the polarity of the differential amplifier 33. Figure 1 In the example shown, a gas detection signal Vgas or a temperature signal Vtemp is supplied to the in-phase input terminal (+), and a reference voltage Vref is supplied to the in-phase input terminal (-). In this case, the higher the concentration of CO2 gas in the measured atmosphere, the higher the level of the differential signal Vdiff.

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

[0059] The control circuit 35 calculates the concentration of CO2 gas, the gas to be measured, based on the differential signal Vdiff_ADC, and generates an output signal Vout representing the concentration of CO2 gas. The calculation formula set within the control circuit 35 can also be used in the calculation of the CO2 gas concentration. Furthermore, the control circuit 35 supplies a power supply voltage Vcc to the sensor unit 10 and the temperature sensor 20 via the sensor element power supply circuit 36, and controls the levels of the heater voltages V13 and V14 supplied to the heaters 13 and 14 respectively via the heater drive circuit 37.

[0060] The control circuit 35 corrects the heater voltages V13 and V14 based on the differential signal Vdiff, obtained by amplifying the difference between the temperature signal Vtemp and the reference voltage Vref. When the CO2 concentration in the measured atmosphere is the normal atmospheric CO2 concentration (e.g., 400 ppm), the control circuit 35 corrects the heater voltages V13 and V14 by heating them for a predetermined time using heaters 13 and 14 until the temperatures of thermistors 11 and 12 reach 150°C and 300°C, respectively. That is, the control circuit 35 changes the levels of the heater voltages V13 and V14 based on the temperature signal Vtemp (more precisely, the differential signal Vdiff_ADC obtained by AD conversion of the difference between the temperature signal Vtemp and the reference voltage Vref), thereby changing the electrical current applied to heaters 13 and 14, and thus changing the heat output of heaters 13 and 14. The level of the reference voltage Vref during ambient temperature measurement is determined by the set value REFT stored in the memory 35c included in the control circuit 35. The REFT setting can also be a fixed value.

[0061] Furthermore, when the level of the differential signal Vdiff_ADC obtained by performing an AD conversion on the difference between the gas detection signal Vgas and the reference voltage Vref is lower than the threshold Vth stored in the memory 35d included in the control circuit 35, the control circuit 35 changes the set value REFG set in the memory 35b to make the level of the differential signal Vdiff_ADC above or closer to the threshold Vth, thereby correcting the level of the reference voltage Vref. When the level of the differential signal Vdiff_ADC is equal to the threshold Vth, the control circuit 35 sets the level of the output signal Vout to a level equivalent to the concentration of CO2 gas in the atmosphere at normal conditions (e.g., 400 ppm). In other words, the threshold Vth is equivalent to the level of the differential signal Vdiff_ADC when the control circuit 35 determines that the concentration of CO2 gas in the atmosphere is at normal conditions (e.g., 400 ppm).

[0062] The operation of the gas sensor 100 of this embodiment will be described in more detail below.

[0063] Figure 3 This is a flowchart illustrating the ambient temperature measurement operation of the gas sensor 100.

[0064] When the gas sensor 100 initiates the ambient temperature measurement operation S10, firstly, the control circuit 35 reads the set value REFT set in the memory 35c and controls the reference voltage generation circuit 32 based on it, thereby setting the level of the reference voltage Vref to the level (Vref_temp) during the ambient temperature measurement operation (step S11). Next, the control circuit 35 supplies the power supply voltage Vcc to the power supply line VL by controlling the sensor element power supply circuit 36 ​​(step S12). As a result, the power supply voltage Vcc is applied to the temperature sensor 20, and a temperature signal Vtemp corresponding to the ambient temperature appears at the connection point N2.

[0065] Next, the control circuit 35 supplies the temperature signal Vtemp to the differential amplifier 33 by controlling the multiplexer 31 (step S13). As a result, the differential amplifier 33 outputs a differential signal Vdiff, which amplifies the potential difference between the temperature signal Vtemp and the reference voltage Vref. In this state, an A / D conversion is performed using the AD converter 34, and the resulting digital value, the differential signal Vdiff_ADC, is supplied to the control circuit 35 (step S14). Then, the control circuit 35 stops the supply of the power supply voltage Vcc by controlling the sensor element power supply circuit 36 ​​(step S15), and calculates the ambient temperature based on the differential signal Vdiff_ADC (step S16). Finally, the control circuit 35 calculates the levels of the heater voltages V13 and V14 based on the calculated ambient temperature (step S17).

[0066] When the ambient temperature measurement action S10 ends, the gas concentration measurement action is then performed.

[0067] Figure 4 This is a flowchart illustrating the first example of the gas concentration measurement operation of the gas sensor 100.

[0068] When the gas sensor 100 initiates a gas concentration measurement operation S20A, firstly, if it is the first gas concentration measurement operation (step S21: Yes), the control circuit 35 reads the initial value INI set in the memory 35a and controls the reference voltage generation circuit 32 accordingly, thereby setting the level of the reference voltage Vref to the level corresponding to the initial value INI of the level (Vref_gas) during the gas concentration measurement operation (step S22). This initial value INI is also used as the initial value of the setting value REFG in the memory 35b. On the other hand, if the gas concentration measurement operation is the second or subsequent operation (step S21: No), the setting value REFG set in the memory 35b is read, and the level of the reference voltage Vref is set accordingly (step S23).

[0069] Next, the control circuit 35 supplies power voltage Vcc to the power supply line VL by controlling the sensor element power supply circuit 36, and supplies heater voltages V13 and V14 to heaters 13 and 14 respectively by controlling the heater drive circuit 37 (step S24). As a result, the power supply voltage Vcc is applied to the sensor unit 10, and a gas detection signal Vgas corresponding to the CO2 gas concentration in the measured atmosphere appears at the connection point N1.

[0070] Next, the control circuit 35 supplies the gas detection signal Vgas to the differential amplifier 33 by controlling the multiplexer 31 (step S25). As a result, the differential amplifier 33 outputs a differential signal Vdiff, which amplifies the potential difference between the gas detection signal Vgas and the reference voltage Vref. In this state, an A / D conversion is performed using the AD converter 34, and the resulting digital value, the differential signal Vdiff_ADC, is supplied to the control circuit 35 (step S26).

[0071] Next, the control circuit 35 compares the level of the differential signal Vdiff_ADC with the threshold Vth set in the memory 35d (step S27). As described above, the threshold Vth corresponds to the level of the differential signal Vdiff_ADC when the control circuit 35 determines that the concentration of CO2 gas, the gas to be measured, is the concentration of CO2 gas in the atmosphere under normal conditions (e.g., 400 ppm). That is, when the concentration of CO2 gas in the atmosphere under normal conditions is 400 ppm, the concentration of CO2 gas in the measured atmosphere is usually not less than 400 ppm. Therefore, as long as no negative drift occurs in the sensor section 10, the concentration of CO2 gas represented by the differential signal Vdiff_ADC should be 400 ppm or more, so the level of the differential signal Vdiff_ADC is usually above the threshold Vth. Here, negative drift refers to the phenomenon that the level of the gas detection signal Vgas decreases over time even when the actual gas concentration is constant.

[0072] Then, if the level of the differential signal Vdiff_ADC is above the threshold Vth (step S27: No), the control circuit 35 stops the supply of power voltage Vcc by controlling the sensor element power supply circuit 36, and stops the supply of heater voltages V13 and V14 by controlling the heater drive circuit 37 (step S29), and calculates the CO2 gas concentration based on the differential signal Vdiff_ADC (step S30). The calculated CO2 gas concentration is output to the outside as an output signal Vout (step S31). Then, it stands by until the next measurement is performed (step S32). In step S27, if the level of the differential signal Vdiff_ADC is above the threshold Vth, the value of the reference voltage Vref is maintained, and the set value REFG in the memory 35b is maintained. In this case, if it is the first gas measurement, the initial value INI is written to the memory 35b as the set value REFG, and the set value REFG is updated.

[0073] On the other hand, if the level of the differential signal Vdiff_ADC is lower than the threshold Vth in step S27 (step S27: Yes), the control circuit 35 applies correction to the set value REFG of the memory 35b to reduce the level of the reference voltage Vref (step S28A). As described above, the threshold Vth corresponds to the level of the differential signal Vdiff_ADC when the control circuit 35 determines that the concentration of CO2 gas, the gas to be measured, is the concentration of CO2 gas in the atmosphere under normal conditions (e.g., 400 ppm). Therefore, a level of the differential signal Vdiff_ADC lower than the threshold Vth means that a negative drift occurs in the sensor unit 10. In step S28A, the control circuit 35 corrects the reference voltage Vref to compensate for such a negative drift.

[0074] The correction amount for the reference voltage Vref can also be the smallest correctable gap. For example, when using... Figure 2 In the case of the reference voltage generation circuit 32 shown in (a), the level of the reference voltage Vref can also be lowered by one bit by decreasing the digital value supplied to the DA converter 32a. In this way, the reference voltage Vref is corrected, and as the level of the reference voltage Vref decreases, the level of the differential signal Vdiff also changes (increases). Along with the correction of the reference voltage Vref, the control circuit 35 updates the set value REFG in the memory 35b to the corrected value of the reference voltage Vref. Then, after correcting the reference voltage Vref, it returns to step S26 to perform A / D conversion using the AD converter 34, and supplies the resulting differential signal Vdiff_ADC to the control circuit 35.

[0075] This process is repeated until the level of the differential signal Vdiff_ADC reaches or exceeds the threshold Vth. If the level of the differential signal Vdiff_ADC reaches or exceeds the threshold Vth (step S27: No), then steps S29 and later are executed. Regarding the update of the memory 35b's setpoint REFG, it can be performed instead of each time in step S28A, and in step S27, if the determination is "No," it is performed based on the level of the final reference voltage Vref. Therefore, even when step S28A is repeatedly executed, the update operation of the memory 35b's setpoint REFG can be set to once.

[0076] The gas concentration measurement operation S20A ends after the above steps. Then, without ending the series of measurements (step S40: No), return to... Figure 3 The ambient temperature measurement operation S10 is shown. In subsequent gas concentration measurement operations S20A, in step S23, the setting value REFG set in memory 35b is read, and the level of the reference voltage Vref is set based on it. That is, the latest setting value REFG (the value of the reference voltage Vref when the differential signal used in the previous gas concentration calculation was generated), maintained in step 27 or updated in step S28A, is used. In other words, the control circuit 35 updates the setting value REFG each time the reference voltage Vref is corrected, therefore, in subsequent gas concentration measurement operations S20A, the level of the reference voltage Vref is set based on the updated latest setting value REFG.

[0077] On the other hand, if the measurement ends (step S40: yes), the series of actions ends.

[0078] Thus, in the first gas concentration measurement operation, when the level of the differential signal Vdiff_ADC is lower than the threshold Vth (step S27: Yes), the reference voltage Vref is corrected in stages before the level of the differential signal Vdiff_ADC becomes higher than the threshold Vth (step S28A). Therefore, the negative drift generated in the sensor unit 10 can be canceled. Moreover, if the correction amount of the reference voltage Vref is kept constant each time, complex calculations are not required.

[0079] Figure 5 This is a flowchart illustrating a modified example of the first example of the gas concentration measurement operation of the gas sensor 100.

[0080] exist Figure 5In the variant shown, steps S34 and S35 are added between steps S27 and S28A. Step S34 determines whether the level of the corrected differential signal Vdiff_ADC is above the threshold Vth, assuming that the level of the reference voltage Vref is reduced by one increment in step S28A. If the determination result is that the level of the corrected differential signal Vdiff_ADC is less than the threshold Vth, step S28A is entered, effectively reducing the level of the reference voltage Vref by one increment. Conversely, if the level of the corrected differential signal Vdiff_ADC is above the threshold Vth, step S35 is entered.

[0081] Step S35 assumes that, in step S28A, the level of the reference voltage Vref is reduced by one increment, and determines whether the level of the differential signal Vdiff_ADC is closer to the threshold Vth. That is, it determines whether the absolute value of the difference between the level of the corrected differential signal Vdiff_ADC and the threshold Vth is less than the absolute value of the difference between the level of the differential signal Vdiff_ADC before correction and the threshold Vth. If the determination result is that the level of the corrected differential signal Vdiff_ADC is closer to the threshold Vth, the process proceeds to step S28A, effectively reducing the level of the reference voltage Vref by one increment. Conversely, if the level of the corrected differential signal Vdiff_ADC moves further away from the threshold Vth, no correction of the differential signal Vdiff_ADC is performed, and the process proceeds to step S29.

[0082] according to Figure 5 In the variation shown, even if the level of the corrected differential signal Vdiff_ADC is above the threshold Vth, further correction of the differential signal Vdiff_ADC is not performed if the level of the corrected differential signal Vdiff_ADC deviates further from the threshold Vth. Therefore, the level of the corrected differential signal Vdiff_ADC is necessarily closer to the threshold Vth than the level of the differential signal Vdiff_ADC before correction, thus preventing measurement errors caused by over-correction of the differential signal Vdiff_ADC. For example, if the CO2 gas concentration indicated by the threshold Vth is 400 ppm and the current CO2 gas concentration indicated by the differential signal Vdiff_ADC is 390 ppm, and the correction interval is 30 ppm, the CO2 gas concentration indicated by the corrected differential signal Vdiff_ADC becomes 420 ppm, and the absolute value of the difference between the level of the differential signal Vdiff_ADC and the threshold Vth increases from 10 ppm to 20 ppm. Therefore, such correction can be avoided.

[0083] Figure 6This is a flowchart illustrating the second example of the gas concentration measurement operation of the gas sensor 100.

[0084] Figure 6 The steps S21~S27 of the gas concentration measurement operation S20B shown in the second example are... Figure 4 The gas concentration measurement operation S20A in the first example shown is the same. Furthermore, in step S27, if the level of the differential signal Vdiff_ADC is above the threshold Vth (step S27: No), then it is the same as... Figure 4 The gas concentration measurement action S20A in the first example shown is performed in the same way as steps S29 to S32.

[0085] Conversely, if the level of the differential signal Vdiff_ADC is lower than the threshold Vth in step S27 (step S27: Yes), the control circuit 35 reduces the level of the reference voltage Vref based on the difference between the level of the differential signal Vdiff_ADC and the threshold Vth, thereby narrowing the difference, i.e., bringing the level of the differential signal Vdiff_ADC closer to the threshold Vth, and updates the set value REFG in the memory 35b to the corrected value of the reference voltage Vref (step S28B). In this case, the reference voltage Vref can be corrected so that the difference becomes zero or higher, i.e., the level of the differential signal Vdiff_ADC becomes higher than the threshold Vth, or the reference voltage Vref can be corrected so that the difference becomes less than zero, i.e., the level of the differential signal Vdiff_ADC becomes less than the threshold Vth. As an example, when the gain of the differential amplifier 33 is set to G, the reference voltage Vref is corrected by reducing the level of the reference voltage Vref by (Vth-Vamp_ADC) / G, and the set value REFG is updated.

[0086] After correcting the reference voltage Vref in this manner, A / D conversion is performed again using AD converter 34, and the resulting differential signal Vdiff_ADC is supplied to control circuit 35 (step S33). In step S33, since the reference voltage Vref has been corrected, the level of the differential signal Vdiff_ADC is closer to the threshold Vth. Then, steps S29 to S32 are executed.

[0087] Thus, in the gas concentration measurement operation S20B of the second example, when the level of the differential signal Vdiff_ADC is lower than the threshold Vth (step S27: Yes), the reference voltage Vref is corrected using the correction amount corresponding to the difference between the level of the differential signal Vdiff_ADC and the threshold Vth (step S28B). Therefore, the negative drift generated in the sensor unit 10 can be canceled in one operation.

[0088] Figure 7 This is a schematic graph illustrating the first effect of the gas sensor 100, where (a) represents an example without negative drift elimination (correction of reference voltage Vref), and (b) represents an example with negative drift elimination (correction of reference voltage Vref). Figure 7 In (a) and (b), symbol A represents the actual CO2 gas concentration (constant), symbol B represents the CO2 gas concentration shown by the output signal Vout, symbol C represents the level of the gas detection signal Vgas, and symbol D represents the level of the reference voltage Vref.

[0089] like Figure 7 As shown in (a), without performing a negative drift elimination operation (correction of the reference voltage Vref) (D = constant), if a negative drift occurs as shown in symbol C, even if the actual CO2 gas concentration A is constant, the level of the gas detection signal Vgas decreases over time. As a result, as shown in symbol B, the CO2 gas concentration indicated by the output signal Vout also decreases. Conversely, when a negative drift elimination operation (correction of the reference voltage Vref) is performed as in this embodiment, as... Figure 7 As shown in (b), when the level of the gas detection signal Vgas decreases over time due to negative drift, the level of the reference voltage Vref also decreases in tandem. Therefore, the output signal Vout can represent the CO2 gas concentration after the effect of negative drift has been eliminated.

[0090] Figure 8 This is a schematic graph illustrating the second effect of the gas sensor 100, where (a) represents an example without negative drift elimination (correction of the reference voltage Vref), and (b) represents an example with negative drift elimination (correction of the reference voltage Vref). Figure 8 In (a) and (b), symbol E represents the correct level (level without drift) of the differential signal Vdiff that should be obtained when the CO2 concentration in the measuring atmosphere is the same as the CO2 concentration in the atmosphere under normal conditions (e.g., 400 ppm), symbol F represents the correct level (level without drift) of the differential signal Vdiff that should be obtained when the CO2 concentration in the measuring atmosphere is higher than the CO2 concentration in the atmosphere under normal conditions (e.g., 5000 ppm), and symbol G represents the actual level of the differential signal Vdiff that is obtained when the CO2 concentration in the measuring atmosphere is the same as the CO2 concentration in the atmosphere under normal conditions (e.g., 400 ppm).

[0091] like Figure 8As shown in (a), without performing a negative drift elimination operation (correction of the reference voltage Vref), if a negative drift occurs as indicated by symbol G, the level of the differential signal Vdiff will decrease over time, even if the CO2 gas concentration in the measured atmosphere remains constant at the normal atmospheric CO2 gas concentration (e.g., 400 ppm). Consequently, the level difference between symbols F and G widens over time, thus reducing the range within the dynamic range of the differential amplifier 33 that is substantially suitable for detecting CO2 gas concentration. Conversely, when a negative drift elimination operation (correction of the reference voltage Vref) is performed as in this embodiment, as shown in (a), the level of the differential signal Vdiff will decrease over time. Figure 8 As shown in (b), the level difference between symbol F and symbol G remains unchanged; therefore, the dynamic range of differential amplifier 33 can be largely ensured to include a range substantially suitable for detecting the concentration of CO2 gas. Additionally, for example, in... Figure 1 In the illustrated embodiment, an AD converter 34 is configured after the differential amplifier 33, and the differential signal Vdiff output from the differential amplifier 33 is input to the AD converter 34. In this case, without performing negative drift cancellation (correction of the reference voltage Vref), as... Figure 8 As shown in (a), the level difference between symbol F and symbol G widens over time; therefore, an AD converter 34 with a large input range is required. Conversely, in the case of performing negative drift elimination (correction of the reference voltage Vref) as in this embodiment, as... Figure 8 As shown in (b), the level difference between symbol F and symbol G does not change, thus the input range required by the AD converter 34 configured in the stage following the differential amplifier 33 can be suppressed to a smaller extent.

[0092] Figure 9 This is a schematic graph illustrating an example of the operation of the gas sensor 100 under conditions of changes in CO2 gas concentration in the measuring atmosphere, where (a) represents the change in the drift of the gas detection signal Vgas, (b) represents the actual change in CO2 gas concentration, (c) represents the change in the levels of the gas detection signal Vgas and the reference voltage Vref, and (d) represents the change in the levels of the differential signal Vdiff_ADC and the output signal Vout.

[0093] exist Figure 9 In (a), the symbol H1 represents the correct level (the level without drift) of the gas detection signal Vgas that should be obtained when the CO2 gas concentration in the measurement atmosphere is constant at the normal atmospheric CO2 gas concentration (e.g., 400 ppm). Figure 9In (a) and (c), the symbol H2 represents the level of the gas detection signal Vgas actually obtained when the CO2 gas concentration in the measurement atmosphere is kept constant at the normal atmospheric CO2 gas concentration (e.g., 400 ppm). Therefore, Figure 9 The difference between symbol H1 and symbol H2 in (a) is equivalent to the negative drift generated in sensor section 10.

[0094] exist Figure 9 In (b), the symbol I represents the concentration of CO2 gas in the measured atmosphere. Figure 9 In (c), the symbol J represents the CO2 gas concentration in the measured atmosphere, such as... Figure 9 The actual gas detection signal Vgas level obtained under the conditions shown in (b) is the level of the reference voltage Vref, where K represents the level of the reference voltage Vref. Figure 9 In (d), the symbols L and M represent the CO2 gas concentration in the measured atmosphere, respectively. Figure 9 The actual levels of the differential signal Vdiff_ADC and the output signal Vout obtained under the conditions shown in (b).

[0095] like Figure 9 As shown in (b), even with variations in CO2 gas concentration, the normal atmospheric CO2 concentration (e.g., 400 ppm) is the lower limit, and the CO2 gas concentration usually does not fall below this. However, if a negative drift occurs in the sensor section 10, then as... Figure 9 As shown in (c), the level of the gas detection signal Vgas gradually becomes lower than the level corresponding to the actual CO2 gas concentration. Therefore, when the actual CO2 gas concentration drops to around 400 ppm, the level of the gas detection signal Vgas drops to a level corresponding to a CO2 gas concentration below 400 ppm. In such a situation, if using... Figures 4-6 As explained, the reference voltage Vref is corrected to reduce its level. Figure 9 The symbol T shown in (c) represents the period during which the correction of the reference voltage Vref is performed. The result is as follows: Figure 9 As shown in (d), the level of the differential signal Vdiff_ADC is corrected to be no lower than the level equivalent to the concentration of CO2 gas in the atmosphere under normal conditions (e.g., 400 ppm), and the final output signal Vout is also linked to it.

[0096] Thus, in this embodiment, since the level of the reference voltage Vref changes according to the negative drift generated in the sensor unit 10, it does not affect the conversion operation of the control circuit 35 from the differential signal Vdiff_ADC to the output signal Vout, and the output signal Vout can be obtained. That is, the control circuit 35 does not perform correction on the differential signal Vdiff_ADC corresponding to the reference voltage Vref to generate the output signal Vout, therefore, the operation for converting the differential signal Vdiff_ADC to the output signal Vout is not complicated.

[0097] Figure 10 This is a circuit diagram showing the structure of the gas sensor 100a in the first modified example.

[0098] like Figure 10 As shown, the gas sensor 100a of the first modified example uses a fixed resistor 15 instead of a thermistor 12 and omits the heater 14. Figure 1 The gas sensor 100 shown is different. Other basic structures are the same. Figure 1 Since the gas sensor 100 shown is identical, the same symbols are used for the same elements, and repeated descriptions are omitted. As illustrated in the gas sensor 100a of the first modified example, the element that serves as the reference side relative to the thermistor 11 for detection does not need to be a thermistor; it can also be a fixed resistor.

[0099] Figure 11 This is a circuit diagram showing the structure of the gas sensor 100b in the second modified example.

[0100] like Figure 11 As shown, the gas sensor 100b in the second modification differs from the previous one in that it omits the temperature sensor 20 and the multiplexer 31. Figure 1 The gas sensor 100 shown is different. Other basic structures are the same. Figure 1 The gas sensor 100 shown is identical; therefore, the same symbols are used to label the same elements, and repeated descriptions are omitted. As illustrated by the gas sensor 100b of the second variation, the temperature sensor 20 may not be necessary when the ambient temperature remains approximately constant.

[0101] Figure 12 This is a circuit diagram showing the structure of the gas sensor 100c in the third modified example.

[0102] like Figure 12 As shown, the gas sensor 100c of the third modification differs from the previous one in that it omits the temperature sensor 20 and the multiplexer 31. Figure 10 The gas sensor 100a shown is different. Other basic structures are the same. Figure 10The gas sensor 100a shown is the same, therefore, the same symbols are used for the same elements, and repeated descriptions are omitted. As illustrated in the gas sensor 100c of the third variation, the element that serves as the reference side relative to the thermistor 11 for detection can also be set as a fixed resistor 15, and the temperature sensor 20 can be omitted.

[0103] The above describes the implementation of the technology involved in this disclosure. However, the technology involved in this disclosure is not limited to the above implementation. Of course, various changes can be made without departing from its spirit, and they are also included in the scope of the technology involved in this disclosure.

[0104] For example, in the above embodiment, a thermistor, which is a resistive element, is used as the temperature sensing element of the sensor section 10, but the present invention is not limited to this. For example, platinum (Pt) or tungsten (W), which is a resistive element, may also be used as the temperature sensing element.

[0105] Furthermore, while the above embodiment uses CO2 as an example for measurement, the present invention is not limited to this. Additionally, the sensor unit used in the present invention is not necessarily a thermally conductive sensor; other types of sensors, such as contact combustion, thermoelectric, semiconductor, electrochemical, solid-state, and optical sensors, can also be used. For example, when the target gas is CO, a contact combustion sensor unit can be used. In this case, the normal concentration of CO is approximately zero; therefore, the threshold Vth can simply be set to the value of the differential signal Vdiff_ADC obtained when the CO concentration is zero.

[0106] The technology disclosed herein includes the following structural examples, but is not limited thereto.

[0107] One aspect of this gas sensor includes: a sensor unit that generates a gas detection signal corresponding to the concentration of the target gas; a differential amplifier that generates a differential signal by amplifying the difference between the gas detection signal and a reference voltage; and a control circuit that generates an output signal representing the concentration of the target gas based on the differential signal. When the level of the differential signal is lower than a threshold value corresponding to the level of the differential signal used to determine that the concentration of the target gas is at its normal concentration, the control circuit corrects the reference voltage to make the level of the differential signal above the threshold value. Therefore, even if the sensor unit experiences negative drift, it can be eliminated.

[0108] Another aspect of the gas sensor disclosed herein includes: a sensor unit that generates a gas detection signal corresponding to the concentration of the target gas; a differential amplifier that generates a differential signal by amplifying the difference between the gas detection signal and a reference voltage; and a control circuit that generates an output signal representing the concentration of the target gas based on the differential signal. When the level of the differential signal is lower than a threshold value corresponding to the level of the differential signal used to determine that the concentration of the target gas is at its normal concentration, the control circuit corrects the reference voltage to bring the level of the differential signal closer to the threshold value. Therefore, even if negative drift occurs in the sensor unit, it can be eliminated.

[0109] In the aforementioned gas sensor, the control circuit may include a memory that updates a setpoint related to the reference voltage when the reference voltage has been corrected. The control circuit determines whether the differential signal is below a threshold value when the reference voltage level is set based on the setpoint stored in the memory. This allows the reference voltage level to follow the time-dependent changes in negative drift.

[0110] In the aforementioned gas sensor, the control circuit may generate an output signal without correcting the differential signal for a reference voltage. This avoids complicating the calculation of gas concentration based on the differential signal.

[0111] In the gas sensor described above, the control circuit can also periodically correct the reference voltage when the differential signal level is below a threshold. This reduces the computational load on the control circuit.

[0112] In the aforementioned gas sensor, the control circuit can also correct the reference voltage based on the difference between the differential signal level and the threshold when the differential signal level is below a threshold. This allows for high-speed correction of the reference voltage.

[0113] In the gas sensor described above, the gas being measured can be CO2, and its normal concentration can be the normal concentration of CO2 in the atmosphere. This provides a CO2 gas sensor that can eliminate negative drift.

Claims

1. A gas sensor, wherein, have: The sensor unit generates a gas detection signal corresponding to the concentration of the gas being measured; A differential amplifier generates a differential signal by amplifying the difference between the gas detection signal and the reference voltage; and The control circuit generates an output signal representing the concentration of the gas being measured, based on the differential signal. If the level of the differential signal is below a threshold, the control circuit corrects the reference voltage so that the level of the differential signal becomes above the threshold, wherein the threshold is equivalent to the level of the differential signal used to determine that the concentration of the gas being measured is at its normal concentration.

2. A gas sensor, wherein, have: The sensor unit generates a gas detection signal corresponding to the concentration of the gas being measured; A differential amplifier generates a differential signal by amplifying the difference between the gas detection signal and the reference voltage; and The control circuit generates an output signal representing the concentration of the gas being measured, based on the differential signal. If the level of the differential signal is below a threshold, the control circuit corrects the reference voltage to bring the level of the differential signal closer to the threshold, wherein the threshold is equivalent to the level of the differential signal used to determine that the concentration of the gas being measured is at its normal concentration.

3. The gas sensor according to claim 1 or 2, wherein, The control circuit includes a memory that updates a set value related to the reference voltage when the reference voltage has been corrected. When the control circuit sets the level of the reference voltage based on the set value stored in the memory, it determines whether the differential signal is lower than the threshold.

4. The gas sensor according to claim 1 or 2, wherein, The control circuit generates the output signal without correcting the differential signal according to the reference voltage.

5. The gas sensor according to claim 1 or 2, wherein, The control circuit calibrates the reference voltage in stages when the level of the differential signal is lower than the threshold.

6. The gas sensor according to claim 1 or 2, wherein, When the level of the differential signal is lower than the threshold, the control circuit corrects the reference voltage based on the difference between the level of the differential signal and the threshold.

7. The gas sensor according to claim 1 or 2, wherein, The gas being measured is CO2 gas, and the normal concentration is the concentration of CO2 gas in the atmosphere under normal conditions.

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