JP2022056487A - Gas sensor - Google Patents

Abstract

To eliminate the influence of a gas that is different from the gas to be detected, and measure the concentration of the gas to be detected.SOLUTION: A gas sensor 10 acquires a detection signal Vout1 that appears on a node N0 while a thermistor Rd1 and a thermistor Rd2 are connected in series and a detection signal Vout2 that appears on a node N0 while a fixed resistor R0 and the thermistor Rd2 are connected in series, and cancels the signal component attributable to the concentration of gases that are not the gas to be detected and are included in the detection signal Vout1, on the basis of the detection signal Vout2, thereby calculating the concentration of the gas to be detected. Thus, it is possible to accurately calculate the concentration of the gas to be detected, irrespective of the concentration of gases that are not the gas to be detected.SELECTED DRAWING: Figure 1

Description

The present invention relates to a gas sensor that detects a gas contained in an atmosphere, and more particularly to a gas sensor that can cancel the influence of a gas different from the gas to be detected.

The gas sensor detects the concentration of the gas to be measured contained in the atmosphere, but an error may occur in the measured value due to a gas different from the gas to be detected contained in the atmosphere. In Patent Document 1, two thermistors whose resistance value changes depending on the concentration of the detection target gas are connected in series, and a correction resistance is connected in parallel to one of the thermistors, so that the gas is different from the detection target gas. A gas sensor capable of canceling the measurement error due to the above is disclosed.

JP-A-2019-60848

However, the gas sensor described in Patent Document 1 may not be able to correctly cancel the measurement error when the concentration of the gas different from the detection target gas is high.

Therefore, an object of the present invention is to provide a gas sensor capable of eliminating the influence of a gas different from the detection target gas with high accuracy.

The gas sensor according to the present invention has a first thermistor in which the resistance value changes with the first sensitivity according to the concentration of the first gas and the resistance value changes with the second sensitivity according to the concentration of the second gas. , The resistance value changes at a third sensitivity lower than the first sensitivity according to the concentration of the first gas, and the resistance value changes at the fourth sensitivity according to the concentration of the second gas. The first detection signal that appears at the connection point between the first thermistor and the second thermistor, and the fixed resistance and the second, with the thermistor, fixed resistance, and the first thermistor and the second thermistor connected in series. The thermistor is provided with a fixed resistance and a signal processing circuit for acquiring a second detection signal appearing at the connection point of the second thermistor in a state where the thermistors are connected in series, and the signal processing circuit is included in the first detection signal. It is characterized in that the concentration of the first gas is calculated by canceling the signal component caused by the concentration of the gas of 2 based on the second detection signal.

According to the present invention, since the second detection signal is acquired by directly connecting the fixed resistance whose resistance value does not change depending on the concentration of the second gas and the second thermistor, the second gas can be obtained. Regardless of the concentration, it is possible to accurately calculate the concentration of the first gas.

In the present invention, the first thermistor is heated to the first temperature by the first heater, and the second thermistor is heated to the second temperature different from the first temperature by the second heater. It doesn't matter. According to this, for example, the first thermistor and the second thermistor can have the same configuration.

The gas sensor according to the present invention further includes a switch circuit for connecting either one of the first thermistor and the fixed resistance to the second thermistor, and the signal processing circuit has the first and second thermistors as the first and second thermistors, respectively. The switch circuit may be switched while it is heated to the temperature of. According to this, the first and second detection signals can be acquired while the first and second thermistors are kept at a constant temperature, respectively.

In the present invention, the signal processing circuit holds a correction coefficient indicating the ratio of the second sensitivity to the fourth sensitivity, and the correction value is calculated by dividing the second detection signal by the correction coefficient, and the first The signal component due to the concentration of the second gas may be canceled by subtracting the correction value from the detection signal of. According to this, even when the second sensitivity and the fourth sensitivity are different, the signal component caused by the concentration of the second gas can be correctly canceled. In this case, the fourth sensitivity may be higher than the second sensitivity. According to this, the signal component caused by the concentration of the second gas can be canceled more accurately.

The gas sensor according to the present invention further includes a third thermistor for detecting the ambient temperature, and the signal processing circuit has a plurality of correction coefficients assigned for each ambient temperature, and the temperature signal obtained from the third thermistor. The correction coefficient to be used may be determined from a plurality of correction coefficients based on the above. According to this, even when the correction coefficient changes depending on the environmental temperature, the signal component caused by the concentration of the second gas can be accurately canceled. In this case, the plurality of correction coefficients have a first correction coefficient corresponding to the case where the environmental temperature is the first temperature and a second correction coefficient corresponding to the case where the environmental temperature is the second temperature. When the ambient temperature indicated by the temperature signal is between the first temperature and the second temperature, the first correction value obtained by the first correction coefficient and the second correction coefficient obtained by the second correction coefficient are used. The correction value actually used may be determined by linearly interpolating the correction value. According to this, the number of correction coefficients held in the signal processing circuit can be reduced.

In the present invention, the signal processing circuit corrects based on the first and second detection signals obtained when the concentration of the second gas is set variously in an environment where the concentration of the first gas is constant. You may generate a coefficient. According to this, it becomes possible to obtain an accurate correction coefficient.

In the present invention, the first gas may be CO ₂ gas and the second gas may be water vapor. This makes it possible to eliminate the influence of the water vapor concentration in detecting the concentration of CO ₂ gas.

As described above, according to the present invention, it is possible to eliminate the influence of a gas different from the detection target gas with high accuracy.

FIG. 1 is a circuit diagram showing a configuration of a gas sensor 10 according to an embodiment of the present invention. FIG. 2 is a graph showing the relationship between the environmental temperature and the control voltages Vmh1 and Vmh2. FIG. 3 is a top view for explaining the configuration of the sensor unit S. FIG. 4 is a cross-sectional view taken along the line AA shown in FIG. FIG. 5 is a flowchart for explaining the operation of the control unit 26. FIG. 6 is a graph showing some examples of the correction coefficient H. FIG. 7 is a graph for explaining a method of calculating the correction value Vhc by linear interpolation. FIG. 8 is a timing diagram showing an example of the waveforms of the control voltages Vmh1 and Vmh2 and the switching signal SEL.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a circuit diagram showing a configuration of a gas sensor 10 according to an embodiment of the present invention.

As shown in FIG. 1, the gas sensor 10 according to the present embodiment includes a sensor unit S and a signal processing circuit 20. Although not particularly limited, the gas sensor 10 according to the present embodiment detects the concentration of CO ₂ gas in the atmosphere, and cancels the measurement error due to the water vapor concentration (absolute humidity) as described later. It is possible to do. In the present specification, the gas to be detected may be referred to as "first gas", and the gas causing noise may be referred to as "second gas". In the present embodiment, the first gas is CO ₂ gas and the second gas is water vapor.

The sensor unit S is a heat conduction type gas sensor for detecting the concentration of CO ₂ gas, which is a detection target gas, and has first to third sensor units S1 to S3. The first sensor unit S1 includes a first thermistor Rd1 and a first heater resistance MH1 for heating the thermistor Rd1. Similarly, the second sensor unit S2 includes a second thermistor Rd2 and a second heater resistance MH2 for heating the second thermistor Rd2. The third sensor unit S3 includes a third thermistor Rd3. The first to third thermistors Rd1 to Rd3 are made of materials having a negative temperature coefficient of resistance, such as composite metal oxides, amorphous silicon, polysilicon, and germanium. The first and second thermistors Rd1 and Rd2 both detect the concentration of CO ₂ gas, but their operating temperatures are different from each other as described later. Further, the third thermistor Rd3 detects the environmental temperature.

As shown in FIG. 1, the first thermistor Rd1 and the second thermistor Rd2 are connected via a switch circuit SW. The switch circuit SW has nodes N0 to N2, and connects the node N0 to the node N1 or N2 based on the switching signal SEL supplied from the signal processing circuit 20. The nodes N0 to N2 are connected to one end of the second thermistor Rd2, the first thermistor Rd1, and the fixed resistance R0, respectively. The other ends of the first thermistor Rd1 and the fixed resistor R0 are connected to the wiring to which the power supply potential Vcc is supplied, and the other ends of the second thermistor Rd2 are connected to the wiring to which the ground potential GND is supplied. As a result, when the node N1 of the switch circuit SW is selected, the first and second thermistors Rd1 and Rd2 are connected in series between Vcc and GND, and the first detection signal Vout1 is connected to the node N0. appear. On the other hand, when the node N2 of the switch circuit SW is selected, the fixed resistance R0 and the second thermistor Rd2 are connected in series between Vcc and GND, and the second detection signal Vout2 appears at the node N0.

The first thermistor Rd1 is heated by the first heater resistance MH1. The heating temperature of the first thermistor Rd1 by the first heater resistance MH1 is, for example, 150 ° C. When CO ₂ gas is present in the measurement atmosphere in a state where the first thermistor Rd1 is heated, the heat dissipation characteristics of the first thermistor Rd1 change according to the concentration thereof. Such a change appears as a change in the resistance value of the first thermistor Rd1. When the heating temperature of the first thermistor Rd1 is 150 ° C., the resistance value of the first thermistor Rd1 changes with the first sensitivity according to the concentration of the CO ₂ gas. Sensitivity is a change in resistance to a change in the concentration of the gas. The first sensitivity has a sensitivity capable of sufficiently changing the potential of the detection signal Vout1 appearing at the connection point between the first thermistor Rd1 and the second thermistor Rd2.

In the state where the first thermistor Rd1 is heated, if water vapor is present in the measurement atmosphere, the heat dissipation characteristics of the first thermistor Rd1 change according to the concentration thereof. When the heating temperature of the first thermistor Rd1 is 150 ° C., the resistance value of the first thermistor Rd1 changes with the second sensitivity according to the absolute humidity.

The second thermistor Rd2 is heated by the second heater resistance MH2. The heating temperature of the second thermistor Rd2 by the second heater resistance MH2 is, for example, 300 ° C. Even if CO ₂ gas is present in the measurement atmosphere with the second thermistor Rd2 heated, the resistance value of the second thermistor Rd2 hardly changes. This is because when the heating temperature of the second thermistor Rd2 is 300 ° C., the resistance value of the second thermistor Rd2 changes with the third sensitivity according to the concentration of the CO ₂ gas, but the third sensitivity is This is because it is significantly lower than the first sensitivity, preferably 1/10 or less of the first sensitivity, and more preferably almost zero. Therefore, even if the concentration of CO ₂ gas changes, the resistance value of the second thermistor Rd2 hardly changes.

In the state where the second thermistor Rd2 is heated, if water vapor is present in the measurement atmosphere, the heat dissipation characteristics of the second thermistor Rd2 change according to the concentration thereof. When the heating temperature of the second thermistor Rd2 is 300 ° C., the resistance value of the second thermistor Rd2 changes with the fourth sensitivity according to the absolute humidity. The fourth sensitivity is higher than the second sensitivity described above.

As described above, when the node N1 of the switch circuit SW is selected, the first thermistor Rd1 and the second thermistor Rd2 are connected in series, and the detection signal Vout1 is output from the connection point. On the other hand, when the node N2 of the switch circuit SW is selected, the fixed resistance R0 and the second thermistor Rd2 are connected in series, and the detection signal Vout2 is output from the connection point. The detection signals Vout1 and Vout2 are input to the signal processing circuit 20. The resistance value of the fixed resistance R0 does not change depending on the concentration of CO ₂ gas or water vapor, and is substantially constant.

The third sensor unit S3 is composed of a third thermistor Rd3, and the third thermistor Rd3 and the fixed resistance R1 are connected in series between the wiring to which the power supply potential Vcc is supplied and the wiring to which the ground potential GND is supplied. Has been done. A temperature signal Vout3 is output from the connection point between the third thermistor Rd3 and the resistance R1. The temperature signal Vout3 is input to the signal processing circuit 20.

The signal processing circuit 20 includes differential amplifiers 21 to 23, an AD converter (ADC) 24, a DA converter (DAC) 25, a control unit 26, and fixed resistors R2 to R4. The differential amplifier 21 is a circuit that compares the detection signals Vout1 and Vout2 with the reference voltage Vref and amplifies the difference. The gain of the differential amplifier 21 is arbitrarily adjusted by the fixed resistances R2 to R4. The amplification signals Vamp1 and Vamp2 output from the differential amplifier 21 are input to the AD converter 24. The amplified signal Vamp1 corresponds to the detection signal Vout1, and the amplified signal Vamp2 corresponds to the detection signal Vout2.

The AD converter 24 digitally converts the amplification signals Vamp1 and Vamp2, and supplies the values to the control unit 26. On the other hand, the DA converter 25 generates a reference voltage Vref by analog-converting the reference signal supplied from the control unit 26, and also supplies the control voltages Vmh1 and Vmh2 supplied to the first and second heater resistors MH1 and MH2. It plays a role in generating. The control voltage Vmh1 is applied to the first heater resistor MH1 via the differential amplifier 22 which is a voltage follower. Similarly, the control voltage Vmh2 is applied to the second heater resistor MH2 via the differential amplifier 23, which is a voltage follower.

The temperature signal Vout3 is digitally converted by the AD converter 24 and supplied to the control unit 26. Inside the control unit 26, a mathematical formula or a table showing the relationship between the environmental temperature and the control voltages Vmh1 and Vmh2 is stored, and the control voltages Vmh1 and Vmh2 are corrected by this. FIG. 2 is a graph showing the relationship between the environmental temperature and the control voltages Vmh1 and Vmh2. As shown in FIG. 2, the control unit 26 corrects the control voltages Vmh1 and Vmh2 so that the level of the control voltages Vmh1 and Vmh2 decreases as the environmental temperature rises. In this way, by changing the levels of the control voltages Vmh1 and Vmh2 according to the current environmental temperature obtained by the temperature signal Vout3, the first and second heater resistances MH1 and MH2 are irrespective of the current environmental temperature. The heating temperature can be set as designed.

Further, a mathematical formula or a table showing the relationship between the environmental temperature and the reference voltage Vref is also stored inside the control unit 26, whereby the reference voltage Vref is corrected based on the environmental temperature.

FIG. 3 is a top view for explaining the configuration of the sensor unit S. Further, FIG. 4 is a cross-sectional view taken along the line AA shown in FIG. The drawings are schematic, and for convenience of explanation, the relationship between the thickness and the plane dimensions, the ratio of the thickness between the devices, and the like are different from the actual structure within the range where the effect of the present embodiment can be obtained. It doesn't matter if you do.

The sensor unit S is a heat-conducting gas sensor that detects the gas concentration based on the change in heat dissipation characteristics according to the concentration of CO ₂ gas, and as shown in FIGS. 3 and 4, the two sensor units S1 and S2 A sensor unit S3 arranged between the sensor unit S1 and the sensor unit S2, and a ceramic package 51 accommodating these sensor units S1 to S3 are provided.

The ceramic package 51 is a box-shaped case with an open upper portion, and a lid 52 is provided on the upper portion. The lid 52 has a plurality of vents 53, which allows CO ₂ gas in the atmosphere to flow into the ceramic package 51. The lid 52 is omitted in FIG. 3 in consideration of the legibility of the drawing.

Although not particularly limited, in the present embodiment, three sensor units S1 to S3 are integrated on a single substrate 61. The substrate 61 is formed with three cavities 61a to 61c corresponding to the three sensor units S1 to S3, respectively.

The substrate 61 protects the insulating films 62 and 63, the heater resistors MH1 and MH2 provided on the insulating films 63, the heater protective films 64 covering the heater resistors MH1 and MH2, and the heater protection films 64 at positions overlapping the cavities 61a to 61c, respectively. It includes thermistors Rd1 to Rd3 and thermistor electrodes 35, 45, 65 provided on the film 64, and a thermistor protective film 66 covering the thermistors Rd1 to Rd3 and the thermistor electrodes 35, 45, 65.

The substrate 61 is not particularly limited as long as it has an appropriate mechanical strength and is a material suitable for fine processing such as etching, and is not particularly limited as long as it is a silicon single crystal substrate, a sapphire single crystal substrate, a ceramic substrate, or a quartz substrate. , A glass substrate or the like can be used. The insulating films 62 and 63 are made of an insulating material such as silicon oxide or silicon nitride. The heater resistors MH1 and MH2 are metal materials made of a material having a relatively high melting point, for example, molybdenum (Mo), platinum (Pt), gold (Au), tungsten (W), tantalum (Ta), palladium (Pd), and the like. Iridium (Ir) or an alloy containing any two or more of them is suitable. Thermistors Rd1 to Rd3 are made of materials having a negative temperature coefficient of resistance, such as composite metal oxides, amorphous silicon, polysilicon, and germanium. Here, the thermistor is used as the resistance temperature detector because the resistance temperature coefficient is larger than that of the platinum resistance temperature detector and the like, so that a large detection sensitivity can be obtained. As the material of the heater protective film 64, the same material as the insulating film 63 can be used.

The thermistor electrodes 35, 45, 65 are a pair of electrodes having a predetermined interval, the thermistor Rd1 is provided between the pair of thermistor electrodes 35, the thermistor Rd2 is provided between the pair of thermistor electrodes 45, and the pair of thermistors. A thermistor Rd3 is provided between the electrodes 65. As a result, the resistance value between the pair of thermistor electrodes 35, 45, 65 is determined by the resistance values of the thermistors Rd1 to Rd3, respectively. The materials of the thermista electrodes 35, 45, 65 include molybdenum (Mo), platinum (Pt), gold (Au), tungsten (W), tantalum (Ta), palladium (Pd), iridium (Ir), or any of these. An alloy containing two or more kinds is suitable.

As shown in FIG. 3, both ends of the heater resistance MH1 are connected to the electrode pads 37a and 37b, respectively, and both ends of the heater resistance MH2 are connected to the electrode pads 47a and 47b, respectively. Further, both ends of the thermistor electrode 35 are connected to the electrode pads 37c and 37d, respectively, both ends of the thermistor electrode 45 are connected to the electrode pads 47c and 47d, respectively, and both ends of the thermistor electrode 65 are connected to the electrode pads 67a and 67b, respectively. .. These electrode pads are connected to the package electrode 54 provided in the ceramic package 51 via the bonding wire 55. The package electrode 54 is connected to the signal processing circuit 20 shown in FIG. 1 via an external terminal 56 provided on the back surface of the ceramic package 51.

As described above, since the sensor unit S3 is arranged on the same chip as the sensor units S1 and S2, the sensor unit S3 measures the environmental temperature substantially equal to the environmental temperature received by the sensor units S1 and S2. Is possible. As a result, very accurate temperature measurement becomes possible, so that the heating temperature by the first and second heater resistors MH1 and MH2 can be made almost as designed. The switch circuit SW and fixed resistors R0 and R1 shown in FIG. 1 may be integrated on the substrate 61, may be housed in the ceramic package 51 separately from the substrate 61, or may be housed in the ceramic package 51 outside the ceramic package 51. It may be provided in.

The above is the configuration of the gas sensor 10 according to the present embodiment. Next, the operation of the gas sensor 10 according to the present embodiment will be described.

The gas sensor 10 according to the present embodiment utilizes the fact that the thermal conductivity of the CO ₂ gas is significantly different from the thermal conductivity of the air, and detects changes in the heat dissipation characteristics of the thermistas Rd1 and Rd2 depending on the concentration of the CO ₂ gas signal Vout1. It is taken out as. However, since the thermal conductivity of the measurement atmosphere changes not only with the concentration of CO ₂ gas but also with the absolute humidity, that is, the concentration of water vapor, the influence of the absolute humidity becomes a measurement error. Therefore, the gas sensor 10 according to the present embodiment acquires the detection signal Vout2 by connecting the fixed resistance R0 and the second thermistor Rd2 in series, and cancels the humidity component included in the detection signal Vout1 based on the detection signal Vout2.

The gas sensor 10 according to the present embodiment sufficiently secures the sensitivity to the concentration of CO ₂ gas (first sensitivity) by heating the first thermistor Rd1 to 150 ° C., while the second thermistor Rd2 is set to 300 ° C. By heating, the sensitivity to the concentration of CO ₂ gas (third sensitivity) is set to almost zero. In reality, even if the heating temperature is 300 ° C, there is a slight sensitivity to the concentration of CO ₂ gas, but it is significantly lower than when the heating temperature is 150 ° C, which is about 1/10 or less. Therefore, it can be almost ignored.

On the other hand, the sensitivity to absolute humidity (second sensitivity) when the heating temperature of the first thermistor Rd1 is 150 ° C. and the sensitivity to absolute humidity when the heating temperature of the second thermistor Rd2 is 300 ° C. (second sensitivity). The sensitivities of 4) are different from each other. That is, since the offset amount of the first thermistor Rd1 due to the absolute humidity and the offset amount of the second thermistor Rd2 due to the absolute humidity are not the same, the first thermistor Rd1 and the second thermistor Rd2 are simply connected in series. The effect of absolute humidity is not canceled by connecting to. Specifically, the second sensitivity is about 120 μV /% RH, while the fourth sensitivity is about 200 μV /% RH. Therefore, simply connecting the first thermistor Rd1 and the second thermistor Rd2 in series, that is, the detection signal Vout1 is superposed with the humidity component.

Therefore, the gas sensor 10 according to the present embodiment acquires the detection signal Vout2 by connecting the fixed resistance R0 and the second thermistor Rd2 in series so that the humidity component included in the detection signal Vout1 can be canceled. While the resistance value of the fixed resistance R0 does not change with absolute humidity and is substantially constant, the second thermistor Rd2 is affected by the absolute humidity. Therefore, when the fixed resistance R0 and the second thermistor Rd2 are connected in series, The level of the detection signal Vout2 appearing at the connection point will be substantially determined by the absolute humidity.

The detection signals Vout1 and Vout2 are supplied to the signal processing circuit 20. The detection signals Vout1 and Vout2 are converted into amplification signals Vamp1 and Vamp2 by the differential amplifier 21, and are input to the control unit 26 via the AD converter 24. The control unit 26 cancels the humidity component by subtracting the humidity component indicated by the amplification signal Vamp2 from the amplification signal Vamp1.

FIG. 5 is a flowchart for explaining the operation of the control unit 26.

First, after performing the initial setting (step S1), the temperature signal Vout3 is acquired (step S2), and the environmental temperature is calculated based on this (step S3). Then, the control voltages Vmh1, Vmh2 and the reference voltage Vref are corrected based on the environmental temperature (step S4), and the corrected control voltages Vmh1, Vmh2 and the reference voltage Vref are output (step S5).

Next, the control unit 26 connects the node N0 and the node N1 by controlling the switch circuit SW using the switching signal SEL (step S6). As a result, the first thermistor Rd1 and the second thermistor Rd2 are connected in series, so that the detection signal Vout1 appears at the node N0. The control unit 26 acquires the amplified signal Vamp1 in this state (step S7). Next, the control unit 26 connects the node N0 and the node N2 by controlling the switch circuit SW using the switching signal SEL (step S8). As a result, the fixed resistance R0 and the second thermistor Rd2 are connected in series, so that the detection signal Vout2 appears at the node N0. The control unit 26 acquires the amplified signal Vamp2 in this state (step S9).

Next, the control unit 26 calculates the correction value Vhc based on the amplified signal Vamp2 (step S10). The correction coefficient H held inside the control unit 26 is used to calculate the correction value Vhc. The correction coefficient H indicates the ratio of the sensitivity of the first thermistor Rd1 to the absolute humidity (second sensitivity) and the sensitivity of the second thermistor Rd2 to the absolute humidity (fourth sensitivity). As a method of generating the correction coefficient H, a detection signal obtained when various absolute humiditys are set in a state where CO ₂ gas does not exist or in an environment where the concentration of CO ₂ gas is a constant atmospheric concentration. Vout1 and Vout2 can be actually measured and the correction coefficient H can be generated based on the actual measurement. When the correction coefficient H differs depending on the environmental temperature, the correction coefficient H may be selected based on the temperature signal Vout3.

Next, the control unit 26 calculates the correction value Vhc using the amplification signal Vamp2 and the correction coefficient H (step S10). The calculation of the correction value Vhc is
Vhc = Vamp2 / H ... (Equation 1)
Can be done by. If the correction coefficient H is the reciprocal, the correction value Vhc can be calculated by multiplying the amplification signal Vamp2 by the correction coefficient H. The correction value Vhc corresponds to the humidity component included in the amplified signal Vamp1.

The correction coefficient H may be a mathematical formula or a table. FIG. 6A shows a first example of the correction coefficient H, and shows a case where the relationship between the amplification signal Vamp2 and the correction value Vhc is linear. In this case, the relationship between the amplified signal Vamp2 and the correction value Vhc can be expressed by a linear expression. FIG. 6B shows a second example of the correction coefficient H, and shows a case where the relationship between the amplification signal Vamp2 and the correction value Vhc is curved. In this case, the relationship between the amplified signal Vamp2 and the correction value Vhc can be expressed by a quadratic equation.

FIG. 6C shows a third example of the correction coefficient H, and shows a case where the relationship between the amplification signal Vamp2 and the correction value Vhc differs depending on the environmental temperature. In this case, a plurality of correction coefficients H are prepared, and the correction coefficient H actually used is selected according to the environmental temperature indicated by the temperature signal Vout3. In the example shown in FIG. 6 (c), the formula or table selected when the environmental temperature is T1, T2, T3 is stored in the control unit 26. Therefore, even if the level of the amplified signal Vamp2 is the same, the obtained correction value Vhc value differs depending on the environmental temperature. For example, when the level of the amplification signal Vamp2 is V1, if the environmental temperature is T1, the value of the correction value Vhc is D1, if the environmental temperature is T2, the value of the correction value Vhc is D2, and if the environmental temperature is T3. For example, the value of the correction value Vhc is D3.

When the environmental temperature does not match any of T1, T2, and T3, for example, when the environmental temperature is between T1 and T2, the correction coefficient H is generated by linear interpolation. For example, as shown in FIG. 7, when the environmental temperature is T1 (20 ° C.), the correction coefficient H is D1 (8.00), and when the environmental temperature is T2 (30 ° C.), the correction coefficient H is D2 (7.90). If the environmental temperature is Tc (27 ° C.) between T1 and T2, the value of the correction coefficient H actually used is 7.93 by linear interpolation shown in the following equation.
H = D1 × (T2-Tc) / (T2-T1) + D2 × (Tc-T1) / (T2-T1) = 7.93 ... (Equation 2)

When the correction value Vhc is determined based on the amplification signal Vamp2 and the correction coefficient H, the corrected value Vamp1'is calculated by subtracting the correction value Vhc from the amplification signal Vamp1 (step S11). in short,
Vamp1'= Vamp1-Vhc ... (Equation 3)
Is. The corrected value Vamp1'corresponds to the value at which the humidity component contained in the amplified signal Vamp1 is canceled. Then, by referring to a mathematical formula or a table showing the relationship between the value Vamp1'stored in the control unit 26 and the CO ₂ gas concentration, an output signal Vco2 showing the CO ₂ gas concentration is generated and output to the outside (step). S12).

As described above, the gas sensor 10 according to the present embodiment acquires the detection signal Vout1 indicating the concentration of CO ₂ gas and the detection signal Vout2 indicating the absolute humidity by switching the switch circuit SW, and the detection signal is based on the detection signal Vout2. Since the humidity component contained in Vout1 is canceled, the concentration of CO ₂ gas can be accurately measured.

FIG. 8 is a timing diagram showing an example of the waveforms of the control voltages Vmh1 and Vmh2 and the switching signal SEL. As shown in FIG. 8, in the present embodiment, after sampling the temperature signal Vout3, the control voltage Vmh1 and the control voltage Vmh2 are set to the activity levels at the same time, so that the first heater resistance MH1 and the second heater resistance MH2 are set. Is heated at the same time. Then, if the detection signals Vout1 and Vout2 are sequentially sampled by switching the switching signal SEL at the timing when the control voltages Vmh1 and Vmh2 are activated, the thermistors Rd1 and Rd2 are respectively maintained at a constant temperature and the detection signals Vout1 are sampled. , Vout2 can be acquired.

Although the preferred embodiment of the present invention has been described above, the present invention is not limited to the above embodiment, and various modifications can be made without departing from the gist of the present invention, and these are also the present invention. Needless to say, it is included in the range.

For example, in the above embodiment, the case where the first gas is CO ₂ gas and the second gas is water vapor has been described as an example, but the present invention is not limited thereto. Further, it is not essential that the sensor unit used in the present invention is a heat conduction type sensor, and another type of sensor such as a contact combustion type sensor may be used.

10 Gas sensor 20 Signal processing circuit 21-23 Differential amplifier 24 AD converter 25 DA converter 26 Control unit 35, 45, 65 Thermistor electrodes 37a to 37d, 47a to 47d, 67a, 67b Electrode pad 51 Ceramic package 52 Lid 53 Vent 54 Package electrode 55 Bonding wire 56 External terminal 61 Substrate 61a to 61c Cavity 62,63 Insulation film 64 Heater protective film 65 Thermistor electrode 66 Thermistor protective film MH1, MH2 Heater resistance N0 to N2 Nodes R0 to R4 Fixed resistance Rd1 to Rd3 Thermistor S, S1 to S3 sensor section SW switch circuit

Claims (9)

A first thermistor in which the resistance value changes with the first sensitivity according to the concentration of the first gas and the resistance value changes with the second sensitivity according to the concentration of the second gas.
The resistance value changes at a third sensitivity lower than the first sensitivity according to the concentration of the first gas, and the resistance value changes at the fourth sensitivity according to the concentration of the second gas. 2 thermistors and
Fixed resistance and
The first detection signal appearing at the connection point between the first thermistor and the second thermistor in a state where the first thermistor and the second thermistor are connected in series, the fixed resistance and the second thermistor. A signal processing circuit for acquiring a second detection signal appearing at the connection point of the fixed resistance and the second thermistor in a state of being connected in series is provided.
The signal processing circuit cancels the signal component caused by the concentration of the second gas contained in the first detection signal based on the second detection signal, thereby reducing the concentration of the first gas. A gas sensor characterized by calculating. The first thermistor is heated to a first temperature by a first heater, and the second thermistor is heated to a second temperature different from the first temperature by a second heater. The gas sensor according to claim 1. A switch circuit for connecting either one of the first thermistor and the fixed resistance to the second thermistor is further provided.
The gas sensor according to claim 2, wherein the signal processing circuit switches the switch circuit in a state where the first and second thermistors are heated to the first and second temperatures, respectively. The signal processing circuit holds a correction coefficient indicating the ratio of the second sensitivity to the fourth sensitivity, and the correction value is calculated by multiplying and dividing the second detection signal by the correction coefficient, and the correction value is calculated. The gas sensor according to any one of claims 1 to 3, wherein the signal component is canceled by subtracting the correction value from the first detection signal. The gas sensor according to claim 4, wherein the fourth sensitivity is higher than the second sensitivity. Further equipped with a third thermistor to detect the environmental temperature,
The signal processing circuit has a plurality of correction coefficients assigned for each environmental temperature, and a correction coefficient to be used from the plurality of correction coefficients is selected based on the temperature signal obtained from the third thermistor. The gas sensor according to claim 4 or 5, wherein the determination is made. The plurality of correction coefficients have a first correction coefficient corresponding to the case where the environmental temperature is the first temperature and a second correction coefficient corresponding to the case where the environmental temperature is the second temperature. When the environmental temperature indicated by the temperature signal is between the first temperature and the second temperature, it is obtained by the first correction value obtained by the first correction coefficient and the second correction coefficient. The gas sensor according to claim 6, wherein the correction value to be actually used is determined by linearly interpolating the second correction value. The signal processing circuit is based on the first and second detection signals obtained when the concentration of the second gas is set variously in an environment where the concentration of the first gas is constant. The gas sensor according to any one of claims 4 to 7, wherein a correction coefficient is generated. The gas sensor according to any one of claims 1 to 8, wherein the first gas is CO ₂ gas and the second gas is water vapor.

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