gas sensor

Abstract

This provides an improved gas sensor that can cancel out the effects of drift. [Solution] The gas sensor 100 includes a sensor unit 10 that generates a gas detection signal Vgas according to the concentration of the 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 the reference voltage Vref, and a control circuit 35 that generates an output signal Vout indicating the concentration of the gas to be measured based on the differential signal Vdiff. If the concentration of the gas to be measured is below a threshold Vth corresponding to the level of the differential signal at which the concentration of the differential signal is determined to be the normal concentration, the control circuit 35 corrects the reference voltage Vref so that the level of the differential signal Vdiff becomes equal to or greater than the threshold Vth, or approaches the threshold Vth.

Description

Technical Field

[0001] The present disclosure relates to a gas sensor, and more particularly to a gas sensor capable of canceling negative drift.

Background Art

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

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the gas sensor described in Patent Document 1, there is a problem that a calculation error occurs in the concentration of the measurement target gas due to the influence of drift caused by changes over time.

[0005] In the present disclosure, an improved gas sensor capable of canceling the influence of drift will be described.

Means for Solving the Problems

[0006] A gas sensor according to one aspect of this disclosure comprises 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 difference signal by amplifying the difference between the gas detection signal and a reference voltage; and a control circuit that generates an output signal indicating the concentration of the gas to be measured based on the difference signal. The control circuit corrects the reference voltage so that the level of the difference signal becomes equal to or greater than the threshold when the concentration of the gas to be measured falls below a threshold corresponding to the level of the difference signal at which the concentration of the gas to be measured is determined to be the normal concentration.

[0007] A gas sensor according to another aspect of this disclosure comprises 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 difference signal by amplifying the difference between the gas detection signal and a reference voltage; and a control circuit that generates an output signal indicating the concentration of the gas to be measured based on the difference signal. The control circuit corrects the reference voltage so that the level of the difference signal approaches the threshold when the concentration of the gas to be measured falls below a threshold level corresponding to the level of the difference signal at which the concentration of the gas to be measured is determined to be the normal concentration. [Effects of the Invention]

[0008] According to this disclosure, a gas sensor capable of canceling negative drift is provided. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is a circuit diagram showing the configuration of a gas sensor 100 according to one embodiment of the technology described herein. [Figure 2] Figure 2 shows an example of the reference voltage generation circuit 32. [Figure 3] Figure 3 is a flowchart illustrating the operation of the gas sensor 100 for measuring ambient temperature. [Figure 4] Figure 4 is a flowchart illustrating a first example of the gas concentration measurement operation by the gas sensor 100. [Figure 5] Figure 5 is a flowchart illustrating a modified example of the first example of the gas concentration measurement operation by the gas sensor 100. [Figure 6] Figure 6 is a flowchart illustrating a second example of the gas concentration measurement operation by the gas sensor 100. [Figure 7] Figure 7 is a schematic graph illustrating the first effect of the gas sensor 100. [Figure 8] Figure 8 is a schematic graph illustrating the second effect of the gas sensor 100. [Figure 9] Figure 9 is a schematic graph illustrating an example of the operation of the gas sensor 100 when the CO2 gas concentration in the measurement atmosphere changes. [Figure 10] Figure 10 is a circuit diagram showing the configuration of the gas sensor 100a according to the first modified example. [Figure 11] Figure 11 is a circuit diagram showing the configuration of the gas sensor 100b according to a second modified example. [Figure 12] Figure 12 is a circuit diagram showing the configuration of the gas sensor 100c according to a third modified example. [Modes for carrying out the invention]

[0010] The embodiments of the technology described herein will be described in detail below with reference to the attached drawings.

[0011] Figure 1 is a circuit diagram showing the configuration of a gas sensor 100 according to one embodiment of the technology described herein.

[0012] As shown in Figure 1, the gas sensor 100 according to this embodiment includes a sensor unit 10 that generates a gas detection signal Vgas according to the concentration of the gas to be measured, a temperature sensor 20 that generates a temperature signal Vtemp according to the ambient temperature, and a signal processing circuit 30. Although not particularly limited, the gas sensor 100 according to this embodiment is a heat conduction type gas sensor for detecting the concentration of CO2 gas in the measurement atmosphere.

[0013] The sensor unit 10 includes thermistors 11 and 12 connected in series between the power supply wiring VL and the 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 between the thermistor 11 and the thermistor 12. The thermistor 11 is a temperature-sensitive element for detection, and the thermistor 12 is a temperature-sensitive element for reference. The thermistors 11 and 12 are resistors whose resistance values change with temperature. Examples of the materials of the thermistors 11 and 12 and the thermistor 21 described later include vanadium oxide, amorphous silicon, polycrystalline silicon, oxides having a spinel-type crystal structure containing manganese, titanium oxide, or yttrium-barium-copper oxide.

[0014] When the thermistor 11, which is a temperature-sensitive element for detection, is heated to a temperature range of 100°C to 230°C, which is a temperature range with high sensitivity to CO2 gas, for example, around 150°C, and CO2 gas is present in the measurement atmosphere, the heat dissipation characteristics of the thermistor 11 change according to its concentration. Such a change appears as a change in the temperature of the thermistor 11, that is, 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 rises. For this reason, when heating the thermistor 11 so that its temperature becomes 150°C when the CO2 gas concentration in the measurement atmosphere is the same as the CO2 gas concentration in the atmosphere during normal times (for example, 400 ppm), if the concentration of CO2 gas present in the measurement atmosphere exceeds the CO2 gas concentration in the atmosphere during normal times, the temperature of the thermistor 11 becomes higher than 150°C according to its concentration. As a result, the higher the CO2 gas concentration in the measurement atmosphere, the lower the resistance value of the thermistor 11.

[0015] On the other hand, even if CO2 gas is present in the measurement atmosphere when the thermistor 12, which is a temperature sensing element for reference, is heated to a temperature range of 250°C to 450°C, for example, around 300°C, which is a temperature range in which CO2 gas has low sensitivity, the heat dissipation characteristics of the thermistor 12 hardly change with respect to its concentration, and the temperature of the thermistor 12 also hardly changes. For this reason, 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 change in resistance value of the thermistor 12 heated to around 300°C due to the concentration of CO2 gas can be almost negligible. As a result, when thermistor 11 is heated to around 150°C and thermistor 12 is heated to around 300°C (that is, when the CO2 gas concentration in the measurement atmosphere is the same as the CO2 gas concentration in the atmosphere under normal conditions, thermistor 11 is heated to 150°C and thermistor 12 is heated to 300°C), a gas detection signal Vgas corresponding to the CO2 gas concentration in the measurement atmosphere appears at the connection point N1 of thermistor 11 and thermistor 12. On the other hand, even if other gases are present in the measurement atmosphere that do not have a significant difference in heat dissipation characteristics when thermistor 11 is heated to around 150°C and when thermistor 12 is heated to around 300°C, the concentration of those gases has almost no effect on the gas detection signal Vgas. This makes it possible for the sensor unit 10 to selectively detect the CO2 gas concentration.

[0016] The temperature sensor 20 includes a thermistor 21 and a resistor 22 connected in series between the power supply wiring VL and ground GND. The temperature signal Vtemp of the temperature sensor 20 appears at the connection point N2 between the thermistor 21 and the resistor 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 to be unaffected, or less affected, by heating from, for example, heaters 13 and 14.

[0017] 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.

[0018] The multiplexer 31 supplies either the gas detection signal Vgas or the temperature signal Vtemp to the differential amplifier 33 under the control of the control circuit 35. The differential amplifier 33 generates a differential signal Vdiff by amplifying the difference (potential difference) between the level of either the gas detection signal Vgas or the temperature signal Vtemp and the level of the reference voltage Vref generated by the reference voltage generation circuit 32. The differential amplifier 33 may generate a differential signal Vdiff by amplifying the difference (potential difference) between the level of either the gas detection signal Vgas or the temperature signal Vtemp and the level of the reference voltage Vref at an arbitrary amplification factor of 1 or more or less than 1. The reference voltage generation circuit 32 may be composed of a DA converter (DAC) 32a that DA-converts 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). In either case, the level of the reference voltage Vref is determined by the initial value INI stored in the memory 35a included in the control circuit 35, the set value REFG stored in the memory 35b, or the set value REFT stored in the memory 35c. The initial value INI may be a fixed value or a switchable value. Also, as will be described later, the set value REFG is updated during the gas concentration measurement operation.

[0019] The polarity of the differential amplifier 33 is not particularly limited, but in the example shown in Fig. 1, the gas detection signal Vgas or the temperature signal Vtemp is supplied to the non-inverting input terminal (+), and the reference voltage Vref is supplied to the inverting input terminal (-). In this case, the level of the differential signal Vdiff increases as the concentration of CO2 gas in the measurement atmosphere increases.

[0020] The differential signal Vdiff output from the differential amplifier 33 is input to the AD converter 34. The AD converter 34 generates a digital differential signal Vdiff_ADC by performing an A / D conversion on the differential signal Vdiff and supplies this to the control circuit 35.

[0021] The control circuit 35 calculates the concentration of the CO2 gas to be measured based on the difference signal Vdiff_ADC and generates an output signal Vout indicating the CO2 gas concentration. The calculation of the CO2 gas concentration may also be done using a calculation formula set within the control circuit 35. 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.

[0022] The control circuit 35 corrects the heater voltages V13 and V14 according to the difference signal Vdiff, which is an amplified difference between the temperature signal Vtemp and the reference voltage Vref. When the CO2 gas concentration in the measurement atmosphere is the normal concentration of CO2 gas in the atmosphere (e.g., 400 ppm), the control circuit 35 corrects the heater voltages V13 and V14 so that the temperatures of thermistors 11 and 12 become 150°C and 300°C, respectively, after heating by heaters 13 and 14 for a predetermined time, regardless of the ambient temperature. In other words, the control circuit 35 changes the power applied to heaters 13 and 14 by changing the levels of heater voltages V13 and V14 according to the temperature signal Vtemp (more precisely, the difference signal Vdiff_ADC obtained by AD conversion of the difference between the temperature signal Vtemp and the reference voltage Vref), thereby changing the amount of heat generated by 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 setting value REFT can be a fixed value.

[0023] Furthermore, if the control circuit 35 determines that the level of the difference signal Vdiff_ADC, obtained by AD conversion of the difference between the gas detection signal Vgas and the reference voltage Vref, is below the threshold Vth stored in the memory 35d included in the control circuit 35, the control circuit 35 corrects the level of the reference voltage Vref by changing the setting value REFG set in memory 35b so that the level of the difference signal Vdiff_ADC becomes equal to or closer to the threshold Vth. When the level of the difference signal Vdiff_ADC is equal to the threshold Vth, the control circuit 35 sets the level of the output signal Vout to a level corresponding to the normal concentration of CO2 gas in the atmosphere (e.g., 400 ppm) when the concentration of the target gas, CO2 gas, is equal to the normal concentration of CO2 gas in the atmosphere (e.g., 400 ppm). In other words, the threshold Vth corresponds to the level of the difference signal Vdiff_ADC when the control circuit 35 determines that the concentration of the target gas, CO2 gas, is the normal concentration of CO2 gas in the atmosphere (e.g., 400 ppm).

[0024] The operation of the gas sensor 100 according to this embodiment will be described in more detail below.

[0025] Figure 3 is a flowchart illustrating the operation of the gas sensor 100 for measuring ambient temperature.

[0026] When the gas sensor 100 starts the ambient temperature measurement operation S10, the control circuit 35 first reads the set value REFT set in the memory 35c and controls the reference voltage generation circuit 32 based on this to set the level of the reference voltage Vref to the level during ambient temperature measurement operation (Vref_temp) (step S11). Next, the control circuit 35 supplies the power supply voltage Vcc to the power supply wiring 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.

[0027] 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 difference signal Vdiff, which is an amplified potential difference between the temperature signal Vtemp and the reference voltage Vref. In this state, the AD converter 34 performs A / D conversion and supplies the resulting digital value, the difference signal Vdiff_ADC, to the control circuit 35 (step S14). Subsequently, the control circuit 35 stops supplying the power supply voltage Vcc by controlling the sensor element power supply circuit 36 ​​(step S15) and calculates the ambient temperature based on the difference signal Vdiff_ADC (step S16). Then, the control circuit 35 calculates the levels of the heater voltages V13 and V14 according to the calculated ambient temperature (step S17).

[0028] Once the ambient temperature measurement operation S10 is completed, the gas concentration measurement operation is then performed.

[0029] Figure 4 is a flowchart illustrating a first example of the gas concentration measurement operation by the gas sensor 100.

[0030] When the gas sensor 100 starts the gas concentration measurement operation S20A, the control circuit 35 first reads the initial value INI set in memory 35a if it is the first gas concentration measurement operation (step S21: YES), and controls the reference voltage generation circuit 32 based on this to set 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 memory 35b. On the other hand, if it is the second or later gas concentration measurement operation (step S21: NO), the setting value REFG set in memory 35b is read, and the level of the reference voltage Vref is set based on this (step S23).

[0031] Next, the control circuit 35 supplies a power supply voltage Vcc to the power supply wiring VL by controlling the sensor element power supply circuit 36, and also supplies heater voltages V13 and V14 to the 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 measurement atmosphere appears at the connection point N1.

[0032] 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 difference signal Vdiff, which is the amplified potential difference between the gas detection signal Vgas and the reference voltage Vref. In this state, the AD converter 34 performs A / D conversion and supplies the resulting digital value, the difference signal Vdiff_ADC, to the control circuit 35 (step S26).

[0033] Next, the control circuit 35 compares the level of the differential signal Vdiff_ADC with the threshold Vth set in 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, which is the gas to be measured, is the concentration of CO2 gas in the atmosphere under normal conditions (for example, 400 ppm). In other words, if the concentration of CO2 gas in the atmosphere under normal conditions is 400 ppm, the concentration of CO2 gas in the measurement atmosphere will not normally fall below 400 ppm. Therefore, unless negative drift occurs in the sensor unit 10, the CO2 gas concentration indicated by the differential signal Vdiff_ADC should be 400 ppm or higher, and the level of the differential signal Vdiff_ADC will normally be equal to or higher than the threshold Vth. Here, negative drift refers to the phenomenon in which the level of the gas detection signal Vgas decreases over time, even if the actual gas concentration remains constant.

[0034] Then, if the level of the difference signal Vdiff_ADC is greater than or equal to the threshold Vth (step S27: NO), the control circuit 35 stops supplying the power supply voltage Vcc by controlling the sensor element power supply circuit 36 ​​and stops supplying the heater voltages V13 and V14 by controlling the heater drive circuit 37 (step S29), and calculates the CO2 gas concentration based on the difference signal Vdiff_ADC (step S30). The calculated CO2 gas concentration is output to the outside as the output signal Vout (step S31). After that, it waits until the next measurement is performed (step S32). In step S27, if the level of the difference signal Vdiff_ADC is greater than or equal to the threshold Vth, the value of the reference voltage Vref is maintained, and the setting value REFG in memory 35b is maintained. In this case, if it is the first gas measurement, the initial value INI is written to memory 35b as the setting value REFG, and the setting value REFG is updated.

[0035] On the other hand, in step S27, if the level of the difference signal Vdiff_ADC is below the threshold Vth (step S27: YES), the control circuit 35 corrects the setting value REFG in memory 35b so that the level of the reference voltage Vref decreases (step S28A). As described above, the threshold Vth corresponds to the level of the difference signal Vdiff_ADC when the control circuit 35 determines that the concentration of CO2 gas, which is the gas to be measured, is the concentration of CO2 gas in the atmosphere under normal conditions (for example, 400 ppm). Therefore, if the level of the difference signal Vdiff_ADC is below the threshold Vth, it means that negative drift is occurring in the sensor unit 10. In step S28A, the control circuit 35 corrects the reference voltage Vref so that this negative drift is canceled out.

[0036] The amount of correction for the reference voltage Vref may be the smallest possible pitch. For example, when using the reference voltage generation circuit 32 shown in Figure 2(a), the level of the reference voltage Vref can be reduced by one pitch by decrementing the digital value supplied to the DA converter 32a by one bit. When the reference voltage Vref is corrected in this way and its level decreases, the level of the difference signal Vdiff also changes (increases). The control circuit 35 updates the setting value REFG in the memory 35b to the corrected value of the reference voltage Vref in accordance with the correction of the reference voltage Vref. After correcting the reference voltage Vref, the process returns to step S26, where the A / D conversion is performed by the AD converter 34, and the resulting difference signal Vdiff_ADC is supplied to the control circuit 35.

[0037] This operation is repeated until the level of the difference signal Vdiff_ADC becomes equal to or greater than the threshold Vth. If the level of the difference signal Vdiff_ADC becomes equal to or greater than the threshold Vth (step S27: NO), then steps S29 and later are executed. Regarding the update of the setting value REFG in memory 35b, instead of performing it every time in step S28A, it may be performed based on the final reference voltage Vref level when NO is determined in step S27. In this case, even if step S28A is executed repeatedly, the update operation of the setting value REFG in memory 35b can be performed only once.

[0038] This completes the gas concentration measurement operation S20A. If the series of measurements is not completed (step S40: NO), the process returns to the ambient temperature measurement operation S10 shown in Figure 3. 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 this value. In other words, the latest setting value REFG (the value of the reference voltage Vref used when generating the difference signal during the previous gas concentration calculation) maintained in step 27 or updated in step S28A is used. Since the control circuit 35 updates the setting value REFG each time the reference voltage Vref is corrected, in subsequent gas concentration measurement operations S20A, the level of the reference voltage Vref is set based on the updated latest setting value REFG. On the other hand, if the measurement is to be terminated (step S40: YES), the series of operations is terminated.

[0039] Thus, in the gas concentration measurement operation according to the first example, if the level of the difference signal Vdiff_ADC is below the threshold Vth (step S27: YES), the reference voltage Vref is corrected in steps until the level of the difference signal Vdiff_ADC becomes equal to or greater than the threshold Vth (step S28A), making it possible to cancel out the negative drift occurring in the sensor unit 10. Moreover, if the amount of correction of the reference voltage Vref each time is kept constant, there is no need to perform complex calculations.

[0040] Figure 5 is a flowchart illustrating a modified example of the first example of the gas concentration measurement operation by the gas sensor 100.

[0041] In the modified example shown in Figure 5, steps S34 and S35 are added between steps S27 and S28A. Step S34 is a step to determine whether the level of the corrected difference signal Vdiff_ADC will be equal to or greater than the threshold Vth if the level of the reference voltage Vref is reduced by one pitch in step S28A. If the result of the determination is that the level of the corrected difference signal Vdiff_ADC is less than the threshold Vth, the process proceeds to step S28A, where the level of the reference voltage Vref is actually reduced by one pitch. On the other hand, if the level of the corrected difference signal Vdiff_ADC is equal to or greater than the threshold Vth, the process proceeds to step S35.

[0042] Step S35 is a step to determine whether the level of the difference signal Vdiff_ADC will approach the threshold Vth if the level of the reference voltage Vref is lowered by one pitch in step S28A. In other words, it is a step to determine whether the absolute value of the difference between the level of the corrected difference signal Vdiff_ADC and the threshold Vth is smaller than the absolute value of the difference between the level of the difference signal Vdiff_ADC and the threshold Vth before correction. If the result of the determination is that the level of the corrected difference signal Vdiff_ADC will approach the threshold Vth, the process proceeds to step S28A, and the level of the reference voltage Vref is actually lowered by one pitch. On the other hand, if the level of the corrected difference signal Vdiff_ADC will move further away from the threshold Vth, the process proceeds to step S29 without correcting the difference signal Vdiff_ADC.

[0043] As shown in the modified example in Figure 5, even if the level of the corrected differential signal Vdiff_ADC is greater than or equal to the threshold Vth, if the level of the corrected differential signal Vdiff_ADC is further away from the threshold Vth, the correction operation of the differential signal Vdiff_ADC is not performed any further. As a result, the level of the corrected differential signal Vdiff_ADC will always be closer to the threshold Vth than the level of the differential signal Vdiff_ADC before correction, thus preventing measurement errors caused by excessive 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, then if the correction pitch is 30 ppm, the CO2 gas concentration indicated by the corrected differential signal Vdiff_ADC will be 420 ppm, and the absolute value of the difference between the level of the differential signal Vdiff_ADC and the threshold Vth will increase from 10 ppm to 20 ppm, thus avoiding such correction.

[0044] Figure 6 is a flowchart illustrating a second example of the gas concentration measurement operation by the gas sensor 100.

[0045] The gas concentration measurement operation S20B in the second example shown in Figure 6 is the same as the gas concentration measurement operation S20A in the first example shown in Figure 4 up to steps S21 to S27. Then, in step S27, if the level of the difference signal Vdiff_ADC is greater than or equal to the threshold Vth (step S27: NO), steps S29 to S32 are executed, similar to the gas concentration measurement operation S20A in the first example shown in Figure 4.

[0046] In contrast, in step S27, if the level of the differential signal Vdiff_ADC is below the threshold Vth (step S27: YES), the control circuit 35 lowers the level of the reference voltage Vref so that the difference between the level of the differential signal Vdiff_ADC and the threshold Vth is reduced, that is, so that the level of the differential signal Vdiff_ADC approaches the threshold Vth, and updates the setting value REFG in memory 35b to the corrected value of the reference voltage Vref (step S28B). In this case, the reference voltage Vref may be corrected so that the difference is zero or greater, that is, so that the level of the differential signal Vdiff_ADC is greater than or equal to the threshold Vth, or the reference voltage Vref may be corrected so that the difference is less than zero, that is, so that the level of the differential signal Vdiff_ADC is less than the threshold Vth. As an example, if the gain of the differential amplifier 33 is G, the reference voltage Vref is corrected so that the level of the reference voltage Vref is reduced by (Vth-Vamp_ADC) / G, and the setting value REFG is updated.

[0047] After correcting the reference voltage Vref in this manner, A / D conversion is performed again by the AD converter 34, and the resulting difference signal Vdiff_ADC is supplied to the control circuit 35 (step S33). In step S33, since the reference voltage Vref has already been corrected, the level of the difference signal Vdiff_ADC approaches the threshold Vth. After that, steps S29 to S32 are executed.

[0048] Thus, in the gas concentration measurement operation S20B according to the second example, if the level of the differential signal Vdiff_ADC is below the threshold Vth (step S27: YES), the reference voltage Vref is corrected using a correction amount corresponding to the difference between the level of the differential signal Vdiff_ADC and the threshold Vth (step S28B). This makes it possible to cancel out the negative drift occurring in the sensor unit 10 in a single operation.

[0049] Figure 7 is a schematic graph illustrating the first effect of the gas sensor 100, where (a) shows an example without negative drift cancellation (correction of the reference voltage Vref), and (b) shows an example with negative drift cancellation (correction of the reference voltage Vref). In Figures 7(a) and (b), code A indicates the actual CO2 gas concentration (constant), code B indicates the CO2 gas concentration indicated by the output signal Vout, code C indicates the level of the gas detection signal Vgas, and code D indicates the level of the reference voltage Vref.

[0050] As shown in Figure 7(a), if negative drift cancellation (correction of the reference voltage Vref) is not performed (D=constant), and negative drift occurs as shown by symbol C, even if the actual CO2 gas concentration A is constant, the level of the gas detection signal Vgas decreases over time, resulting in a decrease in the CO2 gas concentration indicated by the output signal Vout as shown by symbol B. In contrast, when negative drift cancellation (correction of the reference voltage Vref) is performed as in this embodiment, as shown in Figure 7(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 conjunction with it, so the output signal Vout can indicate a CO2 gas concentration where the effect of negative drift has been canceled.

[0051] Figure 8 is a schematic graph illustrating the second effect of the gas sensor 100, where (a) shows an example without negative drift cancellation (correction of the reference voltage Vref), and (b) shows an example with negative drift cancellation (correction of the reference voltage Vref). In Figures 8(a) and (b), the symbol E indicates the correct level of the differential signal Vdiff that should be obtained when the CO2 gas concentration in the measurement atmosphere is the normal concentration of CO2 gas in the atmosphere (e.g., 400 ppm) (the level when there is no drift), the symbol F indicates the correct level of the differential signal Vdiff that should be obtained when the CO2 gas concentration in the measurement atmosphere is higher than the normal concentration of CO2 gas in the atmosphere (e.g., 5000 ppm) (the level when there is no drift), and the symbol G indicates the level of the differential signal Vdiff that is actually obtained when the CO2 gas concentration in the measurement atmosphere is the normal concentration of CO2 gas in the atmosphere (e.g., 400 ppm).

[0052] As shown in Figure 8(a), if negative drift cancellation (correction of the reference voltage Vref) is not performed, and negative drift occurs as shown by the symbol G, the level of the differential signal Vdiff decreases over time, even if the CO2 gas concentration in the measurement atmosphere is constant at the normal atmospheric CO2 gas concentration (e.g., 400 ppm). As a result, the level difference between the symbol F and the symbol G expands over time, creating a range within the dynamic range of the differential amplifier 33 occupied by the negative drift component, and the range of the differential amplifier 33's dynamic range that can be used to detect the CO2 gas concentration is reduced. In contrast, when negative drift cancellation (correction of the reference voltage Vref) is performed as in this embodiment, as shown in Figure 8(b), the level difference between the symbol F and the symbol G does not change, so a large range of the differential amplifier 33's dynamic range that can be used to detect the CO2 gas concentration can be secured. Furthermore, for example, in the embodiment shown in Figure 1, an AD converter 34 is placed after the differential amplifier 33, and the differential signal Vdiff output from the differential amplifier 33 is input to the AD converter 34. In such a case, if negative drift cancellation operation (correction of the reference voltage Vref) is not performed, the level difference between the code F and the code G will increase over time, as shown in Figure 8(a), requiring an AD converter 34 with a large input range. In contrast, if negative drift cancellation operation (correction of the reference voltage Vref) is performed as in this embodiment, the level difference between the code F and the code G will not change, as shown in Figure 8(b), so the input range required for the AD converter 34 placed after the differential amplifier 33 can be kept small.

[0053] Figure 9 is a schematic graph illustrating an example of the operation of the gas sensor 100 when the CO2 gas concentration in the measurement atmosphere changes, where (a) shows the change in the amount of drift of the gas detection signal Vgas, (b) shows the change in the actual CO2 gas concentration, (c) shows the change in the levels of the gas detection signal Vgas and the reference voltage Vref, and (d) shows the change in the levels of the difference signal Vdiff_ADC and the output signal Vout.

[0054] In Figure 9(a), the symbol H1 indicates the correct level 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) (the level without drift). In Figures 9(a) and (c), the symbol H2 indicates the level of the gas detection signal Vgas that is actually obtained when the CO2 gas concentration in the measurement atmosphere is constant at the normal atmospheric CO2 gas concentration (e.g., 400 ppm). Therefore, the difference between the symbols H1 and H2 in Figure 9(a) corresponds to the amount of negative drift occurring in the sensor unit 10.

[0055] In Figure 9(b), the symbol I indicates the CO2 gas concentration in the measurement atmosphere. In Figure 9(c), the symbol J indicates the level of the gas detection signal Vgas actually obtained when the CO2 gas concentration in the measurement atmosphere changes as shown in Figure 9(b), and the symbol K indicates the level of the reference voltage Vref. In Figure 9(d), the symbols L and M indicate the levels of the difference signal Vdiff_ADC and the output signal Vout, respectively, when the CO2 gas concentration in the measurement atmosphere changes as shown in Figure 9(b).

[0056] As shown in Figure 9(b), even when the CO2 gas concentration changes, the normal atmospheric CO2 gas concentration (e.g., 400 ppm) is the lower limit, and normally the CO2 gas concentration does not fall below this. However, if negative drift occurs in the sensor unit 10, as shown in Figure 9(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 falls to around 400 ppm, the level of the gas detection signal Vgas falls to a level corresponding to a CO2 gas concentration of less than 400 ppm. When such a situation occurs, the reference voltage Vref is corrected so that its level decreases, as explained using Figures 4 to 6. The symbol T shown in Figure 9(c) represents the period during which the correction of the reference voltage Vref is performed. As a result, as shown in Figure 9(d), the level of the difference signal Vdiff_ADC is corrected so that it does not fall below a 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 this.

[0057] Thus, in this embodiment, since the level of the reference voltage Vref is changed according to the amount of negative drift occurring in the sensor unit 10, the output signal Vout can be obtained without affecting the conversion operation from the difference signal Vdiff_ADC to the output signal Vout by the control circuit 35. In other words, since the control circuit 35 generates the output signal Vout without performing a correction on the difference signal Vdiff_ADC according to the reference voltage Vref, the calculation for converting from the difference signal Vdiff_ADC to the output signal Vout does not become complicated.

[0058] Figure 10 is a circuit diagram showing the configuration of the gas sensor 100a according to the first modified example.

[0059] As shown in Figure 10, the first modified gas sensor 100a differs from the gas sensor 100 shown in Figure 1 in that a fixed resistor 15 is used instead of the thermistor 12 and the heater 14 is omitted. The other basic configurations are the same as those of the gas sensor 100 shown in Figure 1, so the same elements are denoted by the same reference numerals and redundant explanations are omitted. As illustrated by the first modified gas sensor 100a, the element that serves as the reference for the thermistor 11 used for detection does not necessarily have to be a thermistor; it may be a fixed resistor.

[0060] Figure 11 is a circuit diagram showing the configuration of the gas sensor 100b according to a second modified example.

[0061] As shown in Figure 11, the gas sensor 100b according to the second modification differs from the gas sensor 100 shown in Figure 1 in that the temperature sensor 20 and the multiplexer 31 are omitted. Since the other basic configurations are the same as those of the gas sensor 100 shown in Figure 1, the same reference numerals are used for the same elements, and redundant explanations are omitted. As illustrated by the gas sensor 100b according to the second modification, the temperature sensor 20 is not necessarily required when the ambient temperature is kept almost constant.

[0062] Figure 12 is a circuit diagram showing the configuration of the gas sensor 100c according to a third modified example.

[0063] As shown in Figure 12, the gas sensor 100c according to the third modification differs from the gas sensor 100a shown in Figure 10 in that the temperature sensor 20 and the multiplexer 31 are omitted. Since the other basic configurations are the same as those of the gas sensor 100a shown in Figure 10, the same reference numerals are used for the same elements, and redundant explanations are omitted. As illustrated by the gas sensor 100c according to the third modification, the element that serves as the reference for the thermistor 11 for detection may be a fixed resistor 15, and the temperature sensor 20 may be omitted.

[0064] While embodiments of the technology described herein have been explained above, it goes without saying that the technology described herein is not limited to the embodiments described above, and various modifications are possible without departing from its spirit, and these modifications are also included within the scope of the technology described herein.

[0065] For example, in the above embodiment, a thermistor, which is a resistor, is used as the temperature-sensing element of the sensor unit 10, but the present invention is not limited thereto. For example, platinum (Pt) or tungsten (W), which are resistors, may be used as the temperature-sensing element.

[0066] Furthermore, although the above embodiment was described using the case where the target gas is CO2 gas as an example, the present invention is not limited to this. Also, it is not essential that the sensor unit used in the present invention be a thermal conduction type sensor; other types of sensors such as catalytic combustion type, thermoelectric type, semiconductor type, electrochemical type, solid type, and optical type may be used. For example, when the target gas is CO gas, a catalytic combustion type sensor unit can be used. In this case, since the concentration of CO gas under normal conditions is almost zero, the threshold value Vth should be set to the value of the difference signal Vdiff_ADC obtained when the CO concentration is zero.

[0067] The technology relating to this disclosure includes, but is not limited to, the following configuration examples.

[0068] A gas sensor according to one aspect of this disclosure comprises 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 difference signal by amplifying the difference between the gas detection signal and a reference voltage, and a control circuit that generates an output signal indicating the concentration of the gas to be measured based on the difference signal. The control circuit corrects the reference voltage so that the level of the difference signal becomes equal to or greater than the threshold when the concentration of the gas to be measured falls below a threshold corresponding to the level of the difference signal at which the concentration of the gas to be measured is determined to be the normal concentration. This makes it possible to cancel out negative drift even if it occurs in the sensor unit.

[0069] A gas sensor according to another aspect of this disclosure comprises 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 difference signal by amplifying the difference between the gas detection signal and a reference voltage; and a control circuit that generates an output signal indicating the concentration of the gas to be measured based on the difference signal. The control circuit corrects the reference voltage so that the level of the difference signal approaches the threshold when the concentration of the gas to be measured falls below a threshold corresponding to the level of the difference signal at which the concentration of the gas to be measured is determined to be the normal concentration. This makes it possible to cancel out negative drift even if it occurs in the sensor unit.

[0070] In the gas sensor described above, the control circuit includes a memory in which a setting value for the reference voltage is updated when the reference voltage is corrected. The control circuit may then determine whether the difference signal is below a threshold value after setting the reference voltage level based on the setting value stored in the memory. This makes it possible to make the reference voltage level follow the time-dependent changes in negative drift.

[0071] In the gas sensor described above, the control circuit may generate the output signal without performing any correction to the difference signal according to the reference voltage. This avoids complicating the calculation of the gas concentration based on the difference signal.

[0072] In the gas sensor described above, the control circuit may gradually correct the reference voltage if the level of the differential signal falls below a threshold. This makes it possible to reduce the computational load on the control circuit.

[0073] In the gas sensor described above, the control circuit may correct the reference voltage according to the difference between the difference signal level and the threshold if the difference signal level falls below the threshold. This makes it possible to correct the reference voltage at high speed.

[0074] In the gas sensor described above, the gas to be measured is CO2 gas, and the normal concentration can be the normal concentration of CO2 gas in the atmosphere. This makes it possible to provide a CO2 gas sensor that can cancel negative drift. [Explanation of Symbols]

[0075] 10 Sensor section 11,12 Thermistor 13,14 Heater 15 Fixed resistance 20 Temperature Sensors 21 Thermistor 22 resistors 30 Signal Processing Circuits 31 Multiplexer 32. Reference voltage generation circuit 32a DA converter 33 Differential Amplifier 34 AD converters 35 Control circuits 35a~35d Memory 36 Sensor element power supply circuit 37 Heater drive circuit 100, 100a~100c gas sensor N1, N2 connection point VL power wiring VR1, VR2 Variable resistors

Claims

1. A sensor unit that generates a gas detection signal according to the concentration of the gas to be measured, A differential amplifier that generates a difference signal by amplifying the difference between the gas detection signal and the reference voltage, The system includes a control circuit that generates an output signal indicating the concentration of the gas to be measured based on the difference signal, The control circuit corrects the reference voltage so that the level of the difference signal becomes equal to or greater than the threshold value when the concentration of the gas to be measured falls below a threshold value corresponding to the level of the difference signal used to determine that the concentration is at normal levels. Gas sensor.

2. A sensor unit that generates a gas detection signal according to the concentration of the gas to be measured, A differential amplifier that generates a difference signal by amplifying the difference between the gas detection signal and the reference voltage, The system includes a control circuit that generates an output signal indicating the concentration of the gas to be measured based on the difference signal, The control circuit corrects the reference voltage so that the level of the difference signal approaches the threshold when the concentration of the gas to be measured falls below a threshold corresponding to the level of the difference signal used to determine that the concentration is at normal levels. Gas sensor.

3. The control circuit includes a memory in which a setting value related to the reference voltage is updated when the reference voltage is corrected. The control circuit, with the level of the reference voltage set based on the setting value stored in the memory, determines whether the difference signal is below the threshold value. The gas sensor according to claim 1 or 2.

4. The control circuit generates the output signal without performing any correction on the difference signal according to the reference voltage. The gas sensor according to claim 1 or 2.

5. The control circuit corrects the reference voltage in steps if the level of the difference signal falls below the threshold. The gas sensor according to claim 1 or 2.

6. The control circuit corrects the reference voltage according to the difference between the level of the difference signal and the threshold if the level of the difference signal is below the threshold. The gas sensor according to claim 1 or 2.

7. The gas to be measured is CO ２ It is a gas, and the normal concentration is the same as CO2 in the atmosphere under normal conditions. ２ A gas sensor according to claim 1 or 2, which is the concentration of a gas.

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