WO2019065127A1

WO2019065127A1 - Gas sensor

Info

Publication number: WO2019065127A1
Application number: PCT/JP2018/032832
Authority: WO
Inventors: 海田　佳生; 裕松尾
Original assignee: Tdk株式会社
Priority date: 2017-09-26
Filing date: 2018-09-05
Publication date: 2019-04-04

Abstract

[Problem] To measure the concentration of a gas to be detected while eliminating the influence of a different gas from the gas to be detected. [Solution] The present invention comprises a first thermistor Rd1 that has a resistance that changes with a first sensitivity according to the concentration of a first gas and changes with a second sensitivity according to the concentration of a second gas, a second thermistor Rd2 that is connected in series to the first thermistor and has a resistance that changes with a third sensitivity according to the concentration of the first gas and changes with a fourth sensitivity according to the concentration of the second gas, and a correction resistor R1 that is connected in parallel with the first or second thermistor. In the present invention, the first and second thermistors are connected in series and the potential change according to the concentration of the second gas at the point of connection between the first thermistor and second thermistor is cancelled by the correction resistor, so it is possible, without involving computation processing, to simply and highly accurately remove the influence of the second gas and accurately calculate the concentration of the first gas.

Description

Gas sensor

The present invention relates to a gas sensor for detecting a gas contained in an atmosphere, and more particularly to a gas sensor capable of canceling the influence of a gas different from a gas to be detected.

Although the gas sensor detects the concentration of the measurement target gas contained in the atmosphere, an error may occur in the measurement value due to the gas different from the detection target gas contained in the atmosphere. In Patent Document 1, an oxygen concentration measurement unit and a humidity measurement unit are provided separately from the hydrogen sensor unit that detects hydrogen gas to be detected, and a signal obtained from the hydrogen sensor unit is corrected based on the oxygen concentration and humidity. A method is disclosed.

JP, 2011-133401, A

However, in the method described in Patent Document 1, it is necessary to perform arithmetic processing to calculate the gas concentration. Moreover, since the oxygen concentration measuring unit and the humidity measuring unit are provided outside the hydrogen sensor unit, it is difficult not only to upsize the entire device but also to perform accurate correction.

Therefore, an object of the present invention is to provide a gas sensor capable of highly accurately eliminating the influence of a gas different from the gas to be detected without performing arithmetic processing.

In the gas sensor according to the present invention, the resistance value changes at a first sensitivity according to the concentration of the first gas, and the resistance value changes at a second sensitivity according to the concentration of the second gas. The resistance value changes at a third sensitivity which is connected in series to the first thermistor and which is lower than the first sensitivity in accordance with the concentration of the first gas; A second thermistor whose resistance value changes with a fourth sensitivity different from the sensitivity of 2, and a first thermistor connected in parallel to the first or second thermistor according to the concentration of the second gas And a correction resistor that cancels the potential change of the connection point of the second thermistor.

According to the present invention, the first and second thermistors are connected in series, and the potential change at the connection point of the first thermistor and the second thermistor according to the concentration of the second gas is canceled by the correction resistor. As a result, the influence of the second gas can be easily and accurately removed, and the concentration of the first gas can be accurately calculated, without performing arithmetic processing.

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

In the present invention, the fourth sensitivity may be higher than the second sensitivity, and the correction resistor may be connected in parallel to the second thermistor. According to this, since the fourth sensitivity is effectively reduced, it is possible to match the second sensitivity.

In the present invention, when the second sensitivity is a, the fourth sensitivity is b, and the resistance of the second thermistor heated to the second temperature is Rd2, the resistance R1 of the correction resistor is
R1 = (b / a) × Rd2
It may be defined by According to this, it is possible to almost completely remove the influence of the second gas.

The gas sensor according to the present invention may further comprise a third thermistor disposed between the first thermistor and the second thermistor. According to this, it is possible to reduce the thermal interference because the distance between the first thermistor and the second thermistor increases.

The gas sensor according to the present invention changes the first control voltage supplied to the first heater and the second control voltage supplied to the second heater according to the temperature signal supplied from the third thermistor. May be further provided. According to this, the heating temperature by the first and second heaters can be set as the designed temperature regardless of the current environmental temperature.

A gas sensor according to the present invention receives a common control voltage by receiving a common control voltage, a first heater for heating a first thermistor to a first temperature, a second heater for heating a second thermistor to a second temperature, and the like. And a second amplifier for receiving the common control voltage and applying a second control voltage to the second heater. Absent. According to this, it is possible to reduce the measurement error due to the fluctuation of the power supply potential.

In the present invention, the third sensitivity may be 1/10 or less of the first sensitivity. According to this, it is possible to calculate the concentration of the first gas more accurately.

In the present invention, the first thermistor and the second thermistor may be housed in the same package. According to this, since the measurement conditions of the first thermistor and the second thermistor coincide with each other, the influence of the second gas can be more accurately removed.

In the present invention, the first and second thermistors may constitute a heat conduction sensor. Although it is difficult to obtain a high detection sensitivity and a detection error tends to be large, according to the present invention, it is possible to reduce the detection error caused by a gas different from the gas to be detected. Become.

In the present invention, the first gas may be CO ₂ gas, and the second gas may be water vapor. According to this, it becomes possible to eliminate the influence of humidity in the concentration detection of CO ₂ gas.

Thus, according to the present invention, the influence of a gas different from the gas to be detected can be eliminated with high accuracy without performing arithmetic processing. This makes it possible to measure the concentration of the gas to be detected with high accuracy.

FIG. 1 is a circuit diagram showing a configuration of a gas sensor 10A according to a first embodiment of the present invention. FIG. 2 is a top view for explaining the configuration of the sensor unit S. As shown in FIG. FIG. 3 is a cross-sectional view taken along the line AA shown in FIG. FIG. 4 is a graph showing the relationship between the heating temperature of the thermistors Rd1 and Rd2 and the sensitivity. FIG. 5 is a timing chart showing an example of waveforms of control voltages Vmh1 and Vmh2. FIG. 6 is a graph showing measured values, where (a) shows changes in CO ₂ gas and humidity, and (b) shows changes in detection signal Vout1. FIG. 7 is a circuit diagram showing a configuration of a gas sensor 10B according to a second embodiment of the present invention. FIG. 8 is a circuit diagram showing a configuration of a gas sensor 10C according to a third embodiment of the present invention. FIG. 9 is a circuit diagram showing a configuration of a gas sensor 10D according to a fourth embodiment of the present invention. FIG. 10 is a top view for explaining the configuration of the sensor unit S in the fourth embodiment. FIG. 11 is a cross-sectional view taken along a line BB shown in FIG. FIG. 12 is a timing chart for explaining the timing at which the temperature signal Vout2 is sampled. FIG. 13 is a graph showing the relationship between the environmental temperature and the control voltages Vmh1 and Vmh2.

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 10A according to a first embodiment of the present invention.

As shown in FIG. 1, the gas sensor 10A according to the present embodiment includes a sensor unit S and a signal processing circuit 20. Although not particularly limited, the gas sensor 10A according to the present embodiment detects the concentration of CO ₂ gas in the atmosphere, and as described later, the hardware cancels the measurement error caused by the humidity. It is possible. In the present specification, the gas to be detected may be referred to as “first gas”, and the gas that becomes 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 a first sensor unit S1 and a second sensor unit S2. The first sensor unit S1 includes a first thermistor Rd1 and a first heater resistor MH1 for heating the same. Similarly, the second sensor unit S2 includes a second thermistor Rd2 and a second heater resistor MH2 for heating the same. As shown in FIG. 1, the first and second thermistors Rd1 and Rd2 are connected in series between a line to which the power supply potential Vcc is supplied and a line to which the ground potential GND is supplied. The first and second thermistors Rd1 and Rd2 are made of, for example, a material having a negative temperature coefficient of resistance, such as a composite metal oxide, amorphous silicon, polysilicon, or germanium. The thermistors Rd1 and Rd2 both detect the concentration of the CO ₂ gas, but their operating temperatures are different from each other as described later.

The first thermistor Rd1 is heated by the first heater resistor MH1. The heating temperature of the first thermistor Rd1 by the first heater resistance MH1 is 150 ° C., for example. If CO ₂ gas is present in the measurement atmosphere with the first thermistor Rd 1 heated, the heat dissipation characteristics of the first thermistor Rd 1 change according to the concentration. 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 at a first sensitivity according to the concentration of the CO ₂ gas. The first sensitivity is a sensitivity that can sufficiently change the potential of the detection signal Vout1 that appears at the connection point of 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 radiation characteristics of the first thermistor Rd1 change according to the concentration. When the heating temperature of the first thermistor Rd1 is 150 ° C., the resistance value of the first thermistor Rd1 changes at a second sensitivity depending on the humidity.

The second thermistor Rd2 is heated by the second heater resistor MH2. The heating temperature of the second thermistor Rd2 by the second heater resistance MH2 is 300 ° C., for example. Even if the CO ₂ gas is present in the measurement atmosphere with the second thermistor Rd 2 heated, the resistance value of the second thermistor Rd 2 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 at a 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 substantially zero. For this reason, even if the concentration of CO ₂ gas changes, the resistance value of the second thermistor Rd 2 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. When the heating temperature of the second thermistor Rd2 is 300 ° C., the resistance value of the second thermistor Rd2 changes at the fourth sensitivity according to the humidity. The fourth sensitivity is greater than the second sensitivity described above.

Furthermore, the gas sensor 10A according to the present embodiment includes the correction resistor R1 connected in parallel to the second thermistor Rd2. As described later, the correction resistor R1 is provided to cancel the difference between the sensitivity (the second sensitivity) of the first thermistor Rd1 to the humidity and the sensitivity (the fourth sensitivity) of the second thermistor Rd2 to the humidity. .

As described above, the first thermistor Rd1 and the second thermistor Rd2 are connected in series, and the detection signal Vout1 is output from the connection point. The detection signal Vout1 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 resistors R2 to R4. The differential amplifier 21 is a circuit that compares the detection signal Vout1 with the reference voltage Vref and amplifies the difference. The gain of the differential amplifier 21 is arbitrarily adjusted by the resistors R2 to R4. The amplified signal Vamp output from the differential amplifier 21 is input to the AD converter 24.

The AD converter 24 converts the amplified signal Vamp into a digital signal and supplies the value to the control unit 26. On the other hand, the DA converter 25 converts the reference signal supplied from the control unit 26 into an analog signal to generate the reference voltage Vref, and controls the control voltages Vmh1 and Vmh2 supplied to the first and second heater resistors MH1 and MH2. Play 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.

FIG. 2 is a top view for explaining the configuration of the sensor unit S. As shown in FIG. FIG. 3 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 thickness and planar dimensions, the ratio of thickness between devices, etc. are different from the actual structure within the range where the effects of the present embodiment can be obtained. It does not matter.

The sensor unit S is a heat conduction type gas sensor that detects the gas concentration based on the change of the heat release characteristic according to the concentration of the CO ₂ gas, and as shown in FIGS. 2 and 3, the two sensor units S1 and S2 And a ceramic package 51 for housing the sensor units S1 and S2.

The ceramic package 51 is a box-shaped case having an open top, and a lid 52 is provided on the top. The lid 52 has a plurality of air vents 53 so that the CO ₂ gas in the atmosphere can flow into the ceramic package 51. The lid 52 is omitted in FIG. 2 in consideration of the legibility of the drawing.

The first sensor unit S1 includes a substrate 31, insulating

films

32 and 33 respectively formed on the lower and upper surfaces of the substrate 31, a first heater resistor MH1 provided on the insulating film 33, and a first heater A heater protection film 34 covering the resistance MH1, a first thermistor Rd1 and a thermistor electrode 35 provided on the heater protection film 34, and a thermistor protection film 36 covering the first thermistor Rd1 and the thermistor electrode 35 are provided.

The substrate 31 is not particularly limited as long as it has appropriate mechanical strength and is a material suitable for micro processing such as etching, and a silicon single crystal substrate, a sapphire single crystal substrate, a ceramic substrate, a quartz substrate And glass substrates can be used. The substrate 31 is provided with a cavity 31a at a position overlapping with the first heater resistor MH1 in plan view in order to suppress heat conducted to the substrate 31 by the first heater resistor MH1. The portion from which the substrate 31 is removed by the cavity 31a is called a membrane. If the membrane is configured, the heat capacity is reduced by the amount of thinning of the substrate 31, so heating can be performed with less power consumption.

The insulating

films

32 and 33 are made of an insulating material such as silicon oxide or silicon nitride. In the case of using, for example, silicon oxide as the insulating

films

32 and 33, a film formation method such as a thermal oxidation method or a CVD (Chemical Vapor Deposition) method may be used. The film thickness of the insulating

films

32 and 33 is not particularly limited as long as the insulating property is secured, and may be, for example, about 0.1 to 1.0 μm. In particular, since the insulating film 33 is also used as an etching stop layer when forming the cavity 31 a in the substrate 31, the film thickness may be suitable for achieving the function.

The first heater resistance MH1 is made of a conductive material whose resistivity changes with temperature, and made of a material having a relatively high melting point, for example, molybdenum (Mo), platinum (Pt), gold (Au), tungsten (W), tantalum (Ta), palladium (Pd), iridium (Ir), an alloy containing any two or more of these, and the like are preferable. Moreover, it is preferable that it is a conductive material capable of highly accurate dry etching such as ion milling, and in particular, platinum (Pt) having high corrosion resistance is more preferable as a main component. Further, in order to improve the adhesion to the insulating film 33, it is preferable to form an adhesion layer such as titanium (Ti) on a base of Pt.

A heater protection film 34 is formed on the first heater resistance MH1. As a material of the heater protective film 34, it is desirable to use the same material as the insulating film 33. Since the first heater resistance MH1 repeatedly produces a severe thermal change, rising from normal temperature to 150 ° C. and decreasing again to normal temperature, the insulating film 33 and the heater protective film 34 are also strongly thermally stressed. If it receives continuously, it will lead to destruction such as delamination and a crack. However, if the insulating film 33 and the heater protective film 34 are made of the same material, the material properties of the two are the same, and the adhesion is strong, so that delamination occurs as compared with the case where different materials are used. It becomes difficult to produce destruction such as cracks. When silicon oxide is used as the material of the heater protective film 34, the film may be formed by a method such as a thermal oxidation method or a CVD method. The film thickness of the heater protective film 34 is not particularly limited as long as the insulation with the first thermistor Rd1 and the thermistor electrode 35 is ensured, and may be, for example, about 0.1 to 3.0 μm.

The first thermistor Rd1 is made of a material having a negative temperature coefficient of resistance, such as a composite metal oxide, amorphous silicon, polysilicon, or germanium, and can be formed using a thin film process such as sputtering or CVD. The film thickness of the first thermistor Rd1 may be adjusted according to the target resistance value. For example, if the resistance value (R25) at room temperature is set to about 2 MΩ using a MnNiCo-based oxide, a pair Although depending on the distance between the thermistor electrodes 35, the film thickness may be set to about 0.2 to 1 μm. Here, the reason why a thermistor is used as the temperature sensitive resistance element is that a large detection sensitivity can be obtained because the temperature coefficient of resistance is larger than that of a platinum temperature sensor or the like. In addition, the thin film structure makes it possible to efficiently detect the heat generation of the first heater resistor MH1.

The thermistor electrode 35 is a pair of electrodes having a predetermined distance, and a first thermistor Rd1 is provided between the pair of thermistor electrodes 35. Thus, the resistance value between the pair of thermistor electrodes 35 is determined by the resistance value of the first thermistor Rd1. The material of the thermistor electrode 35 is a conductive substance that can withstand processes such as the film forming process and the heat treatment process of the first thermistor Rd1, and is a material having a relatively high melting point, such as molybdenum (Mo) or platinum (Pt). And the like, gold (Au), tungsten (W), tantalum (Ta), palladium (Pd), iridium (Ir), and an alloy containing any two or more of them are preferable.

The first thermistor Rd1 and the thermistor electrode 35 are covered with a thermistor protective film 36. When the first thermistor Rd1 is brought into contact with a material having reducibility to bring it into a high temperature state, oxygen is taken from the thermistor to cause reduction, which affects the thermistor characteristics. In order to prevent this, it is desirable that the material of the thermistor protective film 36 be an insulating oxide film having no reducibility, such as a silicon oxide film.

As shown in FIG. 2, both ends of the first heater resistor MH1 are connected to

electrode pads

37a and 37b provided on the surface of the thermistor protective film 36, respectively. Further, both ends of the thermistor electrode 35 are respectively connected to

electrode pads

37 c and 37 d provided on the surface of the thermistor protective film 36. The electrode pads 37 a to 37 d are connected to the package electrode 54 provided on 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 through an external terminal 56 provided on the back surface of the ceramic package 51.

As described above, since the first sensor unit S1 has a configuration in which the first heater resistor MH1 and the first thermistor Rd1 are stacked on the substrate 31, the heat generated by the first heater resistor MH1 is generated. Efficiently transmit to the first thermistor Rd1.

Similarly, the second sensor unit S2 includes a substrate 41, insulating

films

42 and 43 respectively formed on the lower and upper surfaces of the substrate 41, a second heater resistor MH2 provided on the insulating film 43, and a second sensor unit S2. A heater protection film 44 covering the second heater resistance MH2, a second thermistor Rd2 and a thermistor electrode 45 provided on the heater protection film 44, and a thermistor protection film 46 covering the second thermistor Rd2 and the thermistor electrode 45; Prepare.

The substrate 41 is made of the same material as the substrate 31 used for the first sensor unit S1, and has the same configuration. That is, the cavity 41a is provided at a position overlapping with the second heater resistance MH2 in plan view, thereby suppressing the conduction of heat by the second heater resistance MH2 to the substrate 41. The materials of the insulating

films

42 and 43 are the same as those of the insulating

films

32 and 33, and an insulating material such as silicon oxide or silicon nitride is used. The thickness of the insulating

films

42 and 43 is the same as that of the insulating

films

32 and 33.

In addition, also for the second heater resistance MH2, the heater protective film 44, the second thermistor Rd2, the thermistor electrode 45, and the thermistor protective film 46, the first heater resistance MH1 used for the first sensor unit S1, the heater protective film 34, the first thermistor Rd1, the thermistor electrode 35, and the thermistor protection film 36 have the same configuration. Both ends of the second heater resistance MH2 are connected to

electrode pads

47a and 47b provided on the surface of the thermistor protective film 46, respectively. Further, both ends of the thermistor electrode 45 are respectively connected to

electrode pads

47 c and 47 d provided on the surface of the thermistor protective film 46. The electrode pads 47 a to 47 d are connected to the package electrode 54 provided on the ceramic package 51 via the bonding wire 55.

A large number of sensor units S1 and S2 each having the above configuration are simultaneously manufactured in a wafer state, separated into pieces by dicing, and fixed to the ceramic package 51 using a die paste (not shown). Thereafter, the electrode pads 37a to 37d and 47a to 47d and the corresponding package electrodes 54 are connected by bonding wires 55 using a wire bonding apparatus. As a material of the bonding wire 55, a metal having a low resistance, such as Au, Al or Cu, is preferable.

Finally, the lid 52 having the vent 53 with the outside air is fixed to the ceramic package 51 using an adhesive resin (not shown) or the like. At this time, when the adhesive resin (not shown) is cured and heated, a substance contained in the adhesive resin is generated as a gas, but the gas is easily released to the outside of the package by the air vent 53. It does not affect S2.

The sensor unit S completed in this manner is connected to the signal processing circuit 20 and the power supply via the external terminal 56. In addition, the correction resistor R1 is built in the signal processing circuit 20, housed in the ceramic package 51, or provided on a circuit board on which the signal processing circuit 20 is mounted.

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

The gas sensor 10A according to this embodiment, the CO ₂ gas thermal conductivity of utilizing the point that differs significantly from the thermal conductivity of the air, the thermistor according to the concentration of CO ₂ gas Rd1, Rd2 detection signal a change in the heat dissipation characteristics of Vout1 It is taken out as. However, the thermal conductivity of the measurement atmosphere changes not only with the concentration of CO ₂ gas but also with humidity, that is, the concentration of water vapor, so the influence of humidity causes a measurement error. Therefore, the gas sensor 10A according to the present embodiment adjusts the resistance value of the correction resistor R1 to the humidity so that the error component due to the humidity of the first thermistor Rd1 and the error component due to the humidity of the second thermistor Rd2 coincide. Cancel the change of the detection signal Vout1 based on the

FIG. 4 is a graph showing the relationship between the heating temperature of the thermistors Rd1 and Rd2 and the sensitivity.

As shown in FIG. 4, when the heating temperature of the thermistors Rd1 and Rd2 is 150 ° C. or less, a sufficiently high sensitivity to the concentration of CO ₂ gas can be obtained, while when the heating temperature exceeds 150 ° C., CO _The sensitivity to the concentration of the _two gases decreases, and when the heating temperature reaches 300 ° C., the sensitivity to the concentration of the CO ₂ gas becomes almost zero. In fact, even if the heating temperature is 300 ° C, there is a slight sensitivity to the concentration of CO ₂ gas, but it is much lower than that when the heating temperature is 150 ° C, about 1/10 or less Because there is, it can be almost ignored.

In consideration of the above points, the gas sensor 10A according to the present embodiment sufficiently secures the sensitivity (first sensitivity) to the concentration of the CO ₂ gas by heating the first thermistor Rd1 to 150 ° C. The sensitivity (third sensitivity) to the concentration of CO ₂ gas is made almost zero by heating the thermistor Rd 2 to 300 ° C. Since the first thermistor Rd1 and the second thermistor Rd2 are connected in series, the level of the detection signal Vout1 indicates the concentration of CO ₂ gas if there is no influence of humidity.

On the other hand, the sensitivity to humidity when the heating temperature of the first thermistor Rd1 is 150 ° C. (second sensitivity) and the sensitivity to humidity when the heating temperature of the second thermistor Rd2 is 300 ° C. (fourth The sensitivities are different from one another. Specifically, the fourth sensitivity is about 200 μV /% RH, while the second sensitivity is about 120 μV /% RH. Therefore, the influence of humidity is reflected on the detection signal Vout1 simply by connecting the first thermistor Rd1 and the second thermistor Rd2 in series.

Therefore, in the gas sensor 10A according to the present embodiment, the correction resistor R1 is connected in parallel to the second thermistor Rd2 so that the change of the detection signal Vout1 according to the humidity is cancelled. Here, assuming that the second sensitivity is a, the fourth sensitivity is b, and the resistance of the second thermistor Rd2 heated to 300 ° C. is Rd2, the resistance of the correction resistor R1 is
R1 = (b / a) × Rd2
By setting to, it is possible to almost cancel the change of the detection signal Vout1 according to the humidity. If applied to the above example,
R1 = (200/120) × Rd2 = (5/3) × Rd2
It should be set to

As a result, the influence of the humidity on the first thermistor Rd1 and the influence on the second thermistor Rd2 effectively coincide with each other, so that the detection signal Vout1 does not change even if the humidity changes. Therefore, the level of the detection signal Vout1 is determined by the concentration of the CO ₂ gas.

FIG. 5 is a timing chart showing an example of waveforms of control voltages Vmh1 and Vmh2. As shown in FIG. 5, in the present embodiment, the control voltage Vmh1 and the control voltage Vmh2 are simultaneously brought to active levels to simultaneously heat the first heater resistance MH1 and the second heater resistance MH2. Then, if the detection signal Vout1 is sampled at the timing when the control voltages Vmh1 and Vmh2 are activated, the concentration of the CO ₂ gas can be measured without performing arithmetic processing for canceling the influence of humidity.

FIG. 6 is a graph showing measured values, where (a) shows changes in CO ₂ gas and humidity, and (b) shows changes in detection signal Vout1. As shown in FIG. 6, when the correction resistance R1 is not used, the level of the detection signal Vout1 largely changes due to the humidity, whereas when the correction resistance R1 is used, the influence of the humidity from the detection signal Vout1 is almost the same. It can be seen that it has been completely canceled.

As described above, in the gas sensor 10A according to the present embodiment, the two thermistors Rd1 and Rd2 having different heating temperatures are connected in series, and the correction resistor R1 is connected in parallel to the second thermistor Rd2. The level of the detection signal Vout1 appearing at the connection point of the first and second thermistors Rd1 and Rd2 accurately represents the concentration of CO ₂ gas without being affected by humidity. Therefore, it is possible to immediately measure the concentration of CO ₂ gas without performing arithmetic processing for canceling the influence of humidity.

FIG. 7 is a circuit diagram showing a configuration of a gas sensor 10B according to a second embodiment of the present invention.

As shown in FIG. 7, in the gas sensor 10B according to the present embodiment, the common control voltage Vmh is commonly supplied to the

differential amplifiers

22 and 23, and the differential amplifier 23 is not connected in voltage follower, and the resistors R5 to R5 are connected. It differs from the gas sensor 10A according to the first embodiment shown in FIG. 1 in that the gain is adjusted using R7. The other configuration is the same as that of the gas sensor 10A according to the first embodiment, and therefore the same components are denoted by the same reference numerals, and the redundant description will be omitted.

The resistors R5 to R7 are elements for adjusting the gain of the differential amplifier 23. For example, the resistances of the resistors R5 to R7 are set to R5 = R6 = R7.
By setting to
Vmh2 = 2 × Vmh1
It can be done. That is, it becomes possible to generate two different control voltages Vmh1 and Vmh2 using the common control voltage Vmh.

Thus, even if the level of common control voltage Vmh temporarily changes due to, for example, fluctuation of power supply potential, both control voltages Vmh1 and Vmh2 simultaneously change in conjunction with common control voltage Vmh. The effects of fluctuations in Vmh2 are offset. Therefore, even if the common control voltage Vmh fluctuates, the level of the detection signal Vout1 does not substantially change. Therefore, according to the present embodiment, the concentration of the CO ₂ gas can be measured more stably.

FIG. 8 is a circuit diagram showing a configuration of a gas sensor 10C according to a third embodiment of the present invention.

As shown in FIG. 8, the gas sensor 10C according to the present embodiment is different from the gas sensor 10B according to the second embodiment shown in FIG. 7 in that the correction resistor R1 is connected in parallel to the first thermistor Rd1. It is different. Since the other configuration is the same as that of the gas sensor 10B according to the second embodiment, the same elements will be denoted by the same reference signs, and redundant description will be omitted.

As illustrated in the present embodiment, when the sensitivity (second sensitivity) to the humidity of the first thermistor Rd1 is higher than the sensitivity (fourth sensitivity) to the humidity of the second thermistor Rd2, the second In order to effectively reduce the sensitivity, the correction resistor R1 may be connected in parallel to the first thermistor Rd1.

FIG. 9 is a circuit diagram showing a configuration of a gas sensor 10D according to a fourth embodiment of the present invention.

As shown in FIG. 9, the gas sensor 10D according to the present embodiment is the first embodiment shown in FIG. 1 in that a third sensor unit S3 and a resistor R8, which are temperature sensors, are added to the sensor unit S. This is different from the gas sensor 10A according to FIG. The third sensor unit S3 includes a third thermistor Rd3. The third thermistor Rd3 and a resistor R8 are connected in series between the line to which the power supply potential Vcc is supplied and the line to which the ground potential GND is supplied. ing. A temperature signal Vout2 is output from the connection point of the third thermistor Rd3 and the resistor R8. The temperature signal Vout2 is supplied to the AD converter 24. The other circuit configuration is the same as that of the gas sensor 10A according to the first embodiment, and therefore the same components are denoted by the same reference numerals and redundant description will be omitted.

FIG. 10 is a top view for explaining the configuration of the sensor unit S in the present embodiment. 11 is a cross-sectional view taken along the line BB shown in FIG. The drawings are schematic, and for convenience of explanation, the relationship between thickness and planar dimensions, the ratio of thickness between devices, etc. are different from the actual structure within the range where the effects of the present embodiment can be obtained. It does not matter.

As shown in FIGS. 10 and 11, in the present embodiment, a third sensor unit S3 is disposed between the first sensor unit S1 and the second sensor unit S2. 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.

The substrate 61 includes insulating

films

62 and 63, a heater protection film 64, and a third thermistor Rd3 and a thermistor electrode 65 provided on the heater protection film 64 at a position overlapping the cavity 61c, and first thermistors Rd1 to Rd3. And a thermistor protective film 66 covering the

thermistor electrodes

35, 45 and 65. As shown in FIG. 10, both ends of the thermistor electrode 65 constituting the third sensor unit S3 are connected to

electrode pads

67a and 67b provided on the surface of the thermistor protective film 66, respectively. The

electrode pads

67 a and 67 b are connected to the package electrode 54 provided on the ceramic package 51 via the bonding wire 55. The other basic configuration is the same as the configuration shown in FIG. 2 and FIG. 3, so the same components are denoted by the same reference numerals and redundant description will be omitted.

The above is the configuration of the gas sensor 10D according to the present embodiment. As described above, in the gas sensor 10D according to the present embodiment, since the three sensor units S1 to S3 are integrated on one substrate 61, the third sensor unit S3, which is a temperature sensor, can be used without increasing the number of parts. It can be added. In addition, by arranging the third sensor unit S3 at the center, the distance between the first sensor unit S1 and the second sensor unit S2 can be separated, so that mutual thermal interference can be reduced. That is, since the first sensor unit S1 and the second sensor unit S2 have different heating temperatures and are simultaneously heated, thermal interference may occur if the distance between the two is short. However, in the present embodiment, since the third sensor unit S3 is disposed between the first sensor unit S1 and the second sensor unit S2, the first sensor unit S1 and the second sensor unit S1 are disposed. The thermal interference during S2 is reduced and more accurate measurement is possible.

FIG. 12 is a timing chart for explaining the timing at which the temperature signal Vout2 is sampled. As shown in FIG. 12, also in the present embodiment, the control voltage Vmh1 and the control voltage Vmh2 are simultaneously set to the active level, so that the first heater resistance MH1 and the second heater resistance MH2 are simultaneously heated. Then, the detection signal Vout1 is sampled at the timing when the control voltages Vmh1 and Vmh2 are activated, and the temperature signal Vout2 is sampled at the timing before the control voltages Vmh1 and Vmh2 are activated. As a result, it is possible to accurately measure the environmental temperature by the third sensor unit S3 without being affected by the heating by the first and second heater resistances MH1 and MH2.

The temperature signal Vout2 is supplied to the AD converter 24 shown in FIG. The temperature signal Vout2 supplied to the AD converter 24 is converted into a digital signal and supplied to the control unit 26. Inside the control unit 26, an equation or a table indicating the relationship between the environmental temperature and the control voltages Vmh1 and Vmh2 is stored, whereby the control voltages Vmh1 and Vmh2 are corrected. FIG. 13 is a graph showing the relationship between the environmental temperature and the control voltages Vmh1 and Vmh2. As shown in FIG. 13, the control unit 26 corrects the control voltages Vmh1 and Vmh2 so that the levels of the control voltages Vmh1 and Vmh2 decrease as the environmental temperature rises. Thus, by changing the levels of control voltages Vmh1 and Vmh2 in accordance with the current environmental temperature obtained by temperature signal Vout2, first and second heater resistances MH1 and MH2 are obtained regardless of the current environmental temperature. The heating temperature by can be made the designed temperature.

As described above, in the gas sensor 10D according to the present embodiment, not only the control voltages Vmh1 and Vmh2 are corrected based on the temperature signal Vout2, but also between the first sensor unit S1 and the second sensor unit S2. Since three sensor units S3 are disposed, the thermal interference between the first sensor unit S1 and the second sensor unit S2 is reduced. This makes it possible to measure the concentration of CO ₂ gas more accurately.

Moreover, since the third sensor unit S3 is disposed on the same chip as the first sensor unit S1 and the second sensor unit S2, the first sensor unit S1 and the second sensor unit S2 receive the third sensor unit S3. The third sensor unit S3 can measure an environmental temperature substantially equal to the ambient temperature. Since this enables very accurate temperature measurement, the heating temperature by the first and second heater resistances MH1 and MH2 can be made almost as designed.

Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. It is needless to say that they are included in the scope.

For example, in the above embodiment, the case where the first gas is CO ₂ gas and the second gas is water vapor was described as an example, but the present invention is not limited to this. In addition, the sensor unit used in the present invention is not necessarily a heat conduction sensor, and may be a contact combustion sensor or another sensor.

10A to 10D Gas sensor 20 Signal processing circuit 21 to 23 Differential amplifier 24 AD converter 25 DA converter 26

Control unit

31, 41, 61

Substrate

31a, 41a, 61a to

61c Cavity

32, 33, 42, 43, 62, 63 Insulating

film

34, 44, 64

heater protection film

35, 45, 65

thermistor electrode

36, 36, 66 thermistor protection film 37a to 37d, 47a to 47d, 67a, 67b electrode pad 51 ceramic package 52 lid 53 air vent 54 package electrode 55 bonding wire 56 External terminals MH1 and MH2 Heater resistance R1 Correction resistances R2 to R8 Resistances Rd1 and Rd3 Thermistor S Sensor unit S1 First sensor unit S2 Second sensor unit S3 Third sensor unit

Claims

A first thermistor whose resistance value changes at a first sensitivity according to the concentration of the first gas, and whose resistance changes at a second sensitivity according to the concentration of the second gas;
The resistance value is changed at a third sensitivity which is connected in series to the first thermistor and which is lower than the first sensitivity according to the concentration of the first gas, and the concentration of the second gas is changed to Accordingly, a second thermistor whose resistance value changes with a fourth sensitivity different from the second sensitivity;
A correction resistor connected in parallel to the first or second thermistor and canceling a potential change at a connection point of the first thermistor and the second thermistor according to the concentration of the second gas; A gas sensor comprising:
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.
The gas sensor according to claim 2, wherein the fourth sensitivity is higher than the second sensitivity, and the correction resistance is connected in parallel to the second thermistor.
Assuming that the second sensitivity is a, the fourth sensitivity is b, and the resistance of the second thermistor heated to the second temperature is Rd2, the resistance R1 of the correction resistor is
R1 = (b / a) × Rd2
The gas sensor according to claim 3, characterized in that:
The gas sensor according to any one of claims 2 to 4, further comprising a third thermistor disposed between the first thermistor and the second thermistor.
A control unit is further provided to change a first control voltage supplied to the first heater and a second control voltage supplied to the second heater according to a temperature signal supplied from the third thermistor. The gas sensor according to claim 5, characterized in that:
A first amplifier that receives a common control voltage and applies a first control voltage to the first heater;
The gas sensor according to any one of claims 2 to 6, further comprising: a second amplifier that receives the common control voltage and applies a second control voltage to the second heater.
The gas sensor according to any one of claims 1 to 7, wherein the third sensitivity is 1/10 or less of the first sensitivity.
The gas sensor according to any one of claims 1 to 8, wherein the first thermistor and the second thermistor are housed in the same package.
The gas sensor according to any one of claims 1 to 9, wherein the first and second thermistors constitute a heat conduction sensor.
The gas sensor according to claim 10, wherein the first gas is a CO 2 gas, and the second gas is a water vapor.