Gas sensor

Abstract

The present disclosure provides a gas sensor capable of more accurately controlling the heating temperature of a heater without using a dedicated temperature sensor unit. A gas sensor (1) is provided with: thermistors (Rd1, Rd2); heater resistors (MH1, MH2) for heating the thermistors (Rd1, Rd2), respectively; and a control circuit (20) that, in a state where the heater resistors (MH1, MH2) are heated, generates an output signal (OUT) indicating the concentration of the gas to be measured on the basis of a detection signal appearing at a connection point between the thermistor (Rd1) and the thermistor (Rd2). The control circuit (20) controls the heating temperature of the heater resistors (MH1, MH2) on the basis of the resistance value of the heater resistor (MH2). As a result, the heating temperature of the heater resistors (MH1, MH2) can be controlled without using a dedicated temperature sensor unit.

Description

Technical Field

[0001] The present disclosure relates to a gas sensor. Background Art

[0002] A gas sensor is disclosed in Patent Document 1, which calculates the concentration of a gas to be measured based on the level of a detection signal that appears at the connection point of a first thermistor and a second thermistor connected in series. The gas sensor described in Patent Document 1 has a temperature sensor section including a third thermistor, and controls the heating temperature of the thermistor based on a temperature signal output from the temperature sensor section.

[0003] Prior Art Documents

[0004] Patent Documents

[0005] Patent Document 1: Japanese Patent Application Laid-Open No. 2021-089155 Summary of the Invention

[0006] Technical Problem to be Solved by the Invention

[0007] However, in the gas sensor described in Patent Document 1, since the temperature sensor section uses a third thermistor, the number of required components increases. In addition, since the resistance temperature coefficient of the thermistor is non-linear, the temperature sensor section generates a measurement error.

[0008] In the present disclosure, a gas sensor that can perform accurate temperature measurement with fewer components and thereby can more accurately control the heating temperature of a heater will be described.

[0009] Means for Solving the Technical Problem

[0010] The gas sensor of the present disclosure includes: a first thermistor and a second thermistor connected in series; a first heater and a second heater that respectively heat the first thermistor and the second thermistor; and a control circuit that generates an output signal representing the concentration of the gas to be measured based on a detection signal that appears at the connection point of the first thermistor and the second thermistor in a state where the first heater and the second heater are heated, and the control circuit controls the heating temperature of the first heater and the second heater based on the resistance value of the first heater or the second heater.

[0011] Effects of the Invention

[0012] According to the present disclosure, it is possible to provide a gas sensor that can perform accurate temperature measurement with fewer components and thereby can more accurately control the heating temperature of a heater. Brief Description of the Drawings

[0013] Figure 1It is a circuit diagram showing the structure of the gas sensor 1 which represents the first embodiment of the technology of the present disclosure.

[0014] Figure 2 It is a graph showing the relationship between the temperature and the resistance value of the heater resistors MH1 and MH2.

[0015] Figure 3 It is a graph showing the temperature characteristics of the thermistors Rd1 and Rd2.

[0016] Figure 4 It is a graph showing the relationship between the temperature of the thermistors Rd1 and Rd2 and the sensitivity to CO2 gas.

[0017] Figure 5 It is a circuit diagram showing the structure of the gas sensor 2 which represents the second embodiment of the technology of the present disclosure.

[0018] Figure 6 It is a circuit diagram showing the structure of the gas sensor 3 which represents the third embodiment of the technology of the present disclosure.

[0019] Figure 7 It is a circuit diagram showing the structure of the gas sensor 4 which represents the fourth embodiment of the technology of the present disclosure.

[0020] Description of symbols

[0021] 1 - 4 Gas sensors

[0022] 20 Control circuit

[0023] 21 AD converter

[0024] 22, 23 Variable power supplies

[0025] 24 MPU

[0026] 25 Power supply

[0027] 26 Multiplexer

[0028] 27, 28 Variable current sources

[0029] A1, A2 Amplifiers

[0030] MH1, MH2 Heater resistors

[0031] R1 Fixed resistor

[0032] Rd1, Rd2 Thermistors. Detailed implementation manners

[0033] Hereinafter, embodiments of the technology according to the present disclosure will be described in detail with reference to the drawings.

[0034] Figure 1This is a circuit diagram showing the structure of a gas sensor 1 according to a first embodiment of the technology of the present disclosure.

[0035] As Figure 1 shown, the gas sensor 1 of the first embodiment includes: thermistors Rd1 and Rd2, heater resistors MH1 and MH2 that heat the thermistors Rd1 and Rd2 respectively; and a control circuit 20 that controls the heater resistors MH1 and MH2. The gas sensor 1 of the present embodiment is a heat conduction type gas sensor for detecting the concentration of CO2 gas in the atmosphere, but is not particularly limited.

[0036] The thermistors Rd1 and Rd2 are detection elements made of materials having a negative resistance temperature coefficient, such as composite metal oxides, amorphous silicon, polycrystalline silicon, and germanium. Both the thermistors Rd1 and Rd2 are used to detect the concentration of CO2 gas, but as will be described later, their operating temperatures are different from each other. Here, the thermistor Rd1 constitutes the detection element, and the thermistor Rd2 constitutes the reference element. The thermistor Rd1 and the thermistor Rd2 are connected in series between a power supply 25 that supplies a power supply potential VDDS and the ground, and the detection signal that appears at the connection point between the two is supplied to the control circuit 20.

[0037] The control circuit 20 includes an AD converter (ADC) 21, a variable power supply 22, a variable power supply 23, an MPU 24, a power supply 25, a multiplexer 26, and a fixed resistor R1. The fixed resistor R1 is connected in series with the heater resistor MH2, and the temperature signal that appears at the connection point between them is supplied to the multiplexer 26. The multiplexer 26 supplies either the detection signal that appears at the connection point between the thermistor Rd1 and the thermistor Rd2 or the temperature signal that appears at the connection point between the fixed resistor R1 and the heater resistor MH2 to the AD converter 21 under the control of the MPU 24. The AD converter 21 performs AD conversion on the detection signal or the temperature signal and supplies the resulting digital value to the MPU 24.

[0038] The MPU 24 selects the detection signal that appears at the connection point between the thermistor Rd1 and the thermistor Rd2 during the gas measurement operation, and selects the temperature signal that appears at the connection point between the fixed resistor R1 and the heater resistor MH2 during the resistance measurement operation. Multiple AD converters 21 may be provided instead of using the multiplexer 26. Thereby, the gas measurement operation and the resistance measurement operation can be performed simultaneously. The MPU 24 controls the variable power supplies 22 and 23 based on the temperature signal obtained during the resistance measurement operation.

[0039] Figure 2 This is a graph showing the relationship between the temperature and the resistance value of the heater resistors MH1 and MH2. As Figure 2As shown, the heater resistors MH1 and MH2 have a positive resistance temperature coefficient where the resistance value increases as the temperature rises. Moreover, different from a thermistor, the relationship between temperature and the resistance value is linear. As a conductive material having such characteristics, metals such as platinum can be cited. Also, in the present embodiment, since the fixed resistor R1 is connected in series with the heater resistor MH2, the MPU 24 can measure the resistance value of the heater resistor MH2 by performing an operation based on the temperature signal that appears at this connection point and the output voltage of the variable power supply 22. Further, by converting the resistance value of the heater resistor MH2 into temperature, the current temperature of the heater resistor MH2 can be accurately measured. Here, since the heater resistor MH2 and the thermistor Rd2 are arranged extremely close to each other, the temperature of the thermistor Rd2 can be regarded as being substantially the same as the temperature of the heater resistor MH2. Thus, in the resistance measurement operation, by measuring the resistance value of the heater resistor MH2, the temperature of the thermistor Rd2 can be measured.

[0040] On the other hand, in the gas measurement operation, under the control of the MPU 24, the heater resistor MH1 is heated to 150°C, and the heater resistor MH2 is heated to 300°C. Regarding the above control, it is performed by controlling the output voltages output from the variable power supplies 22 and 23 based on the temperature signal obtained from the resistance measurement operation. That is, regarding the variable power supply 23, its output voltage is controlled so that the temperature signal indicates that the heater resistor MH2 is at 300°C. Regarding the variable power supply 22, its output voltage is controlled by performing a calculation based on the characteristics shown, so as to heat the heater resistor MH1 to 150°C. Figure 2 As shown, the temperature characteristics of the thermistors Rd1 and Rd2 are different from each other, and are designed such that the resistance value of the thermistor Rd1 heated to 150°C is close to the resistance value of the thermistor Rd2 heated to 300°C. In the example shown, the resistance value of the thermistor Rd1 heated to 150°C is 5.1 kΩ, and the resistance value of the thermistor Rd2 heated to 300°C is 4.0 kΩ. The resistance value of the thermistor Rd1 heated to 150°C and the resistance value of the thermistor Rd2 heated to 300°C may also be substantially the same.

[0041] As Figure 3 shown, the temperature characteristics of the thermistors Rd1 and Rd2 are different from each other, and are designed such that the resistance value of the thermistor Rd1 heated to 150°C is close to the resistance value of the thermistor Rd2 heated to 300°C. In Figure 3 the example shown, the resistance value of the thermistor Rd1 heated to 150°C is 5.1 kΩ, and the resistance value of the thermistor Rd2 heated to 300°C is 4.0 kΩ. The resistance value of the thermistor Rd1 heated to 150°C and the resistance value of the thermistor Rd2 heated to 300°C may also be substantially the same.

[0042] Figure 4 is a graph showing the relationship between the temperature of the thermistors Rd1 and Rd2 and the sensitivity to CO2 gas. As Figure 4As shown, the sensitivities of the thermistors Rd1 and Rd2 to CO2 gas vary greatly according to temperature. In the temperature range below 40°C or above 300°C, the sensitivities of the thermistors Rd1 and Rd2 to CO2 gas are approximately zero. In contrast, the sensitivities of the thermistors Rd1 and Rd2 to CO2 gas become maximum at a state of about 150°C.

[0043] Therefore, when the thermistor Rd1 as a detection element is heated to 150°C and there is CO2 gas in the measurement atmosphere, the heat dissipation characteristics of the thermistor Rd1 change according to its concentration. This change is manifested as a change in the resistance value of the thermistor Rd1. On the other hand, when the thermistor Rd2 as a reference element is heated to 300°C and there is CO2 gas in the measurement atmosphere, the heat dissipation characteristics of the thermistor Rd2 hardly change according to its concentration. Therefore, the change in the resistance value of the thermistor Rd2 caused by the concentration of CO2 gas is sufficiently small compared to the change in the resistance value of the thermistor Rd1 heated to 150°C caused by the concentration of CO2 gas. The change in the resistance value of the thermistor Rd2 caused by the concentration of CO2 gas can also be almost negligible.

[0044] Thus, when the thermistors Rd1 and Rd2 are heated, the level of the detection signal that appears at their connection point changes according to the concentration of CO2 gas in the measurement atmosphere. The detection signal is supplied to the MPU 24 via the AD converter 21, and the MPU 24 generates an output signal OUT representing the concentration of CO2 gas based on this.

[0045] In this way, the gas sensor 1 of the present embodiment does not use a dedicated temperature sensor section, and calculates the temperature of the thermistor Rd2 by measuring the resistance value of the heater resistor MH2, so the number of components can be reduced. Moreover, the materials used for the heater resistors MH1 and MH2 have a linear resistance temperature coefficient, so measurement errors can also be suppressed and accurate temperature measurement can be performed. In addition, in the present embodiment, since the resistance value of the heater resistor MH2 heated to a higher temperature is measured, the measurement error can be further reduced. In particular, the heat dissipation characteristics of the thermistor Rd1 heated to 150°C change based on the concentration of CO2 gas in the atmosphere, so the relationship between the temperature of the heater resistor MH1 and the temperature of the thermistor Rd1 changes slightly according to the concentration of CO2 gas in the atmosphere. However, for the thermistor Rd2 heated to 300°C, the heat dissipation characteristics based on the concentration of CO2 gas in the atmosphere hardly change, so the relationship between the temperature of the heater resistor MH2 and the temperature of the thermistor Rd2 hardly changes according to the concentration of CO2 gas in the atmosphere. Therefore, by measuring the resistance value of the heater resistor MH2, more accurate temperature measurement can be performed.

[0046] In addition, in the present embodiment, the resistance value of the heater resistor MH2 is measured, and the voltages output from the variable power supplies 22 and 23 are adjusted based on this. However, the resistance values of the heater resistors MH1 and MH2 may be measured respectively, and the voltage output from the variable power supply 22 may be adjusted based on the resistance value of the heater resistor MH1, and the voltage output from the variable power supply 23 may be adjusted based on the resistance value of the heater resistor MH2.

[0047] Figure 5 is a circuit diagram showing the structure of the gas sensor 2 according to the second embodiment of the technology of the present disclosure.

[0048] As Figure 5 shown, in the gas sensor 2 of the second embodiment, variable current sources 27 and 28 are used instead of the variable power supplies 22 and 23, which is different from the gas sensor 1 of the first embodiment in this regard. Accordingly, the output current from the variable current source 27 is supplied to the heater MH1, and the output current from the variable current source 28 is supplied to the heater MH2. Other basic structures are the same as those of the gas sensor 1 of the first embodiment, so the same reference numerals are assigned to the same elements, and repeated descriptions are omitted.

[0049] In the present embodiment, the heater resistor MH1 is heated to 150°C and the heater resistor MH2 is heated to 300°C by controlling the amounts of current supplied from the variable current sources 27 and 28 to the heater resistors MH1 and MH2 respectively based on the temperature signal. As exemplified in the present embodiment, the adjustment of the heating temperatures of the heater resistors MH1 and MH2 can also be performed by current control. In this case, the fixed resistor R1 is not essential and may be omitted.

[0050] Figure 6 is a circuit diagram showing the structure of the gas sensor 3 according to the third embodiment of the technology of the present disclosure.

[0051] As Figure 6 shown, the gas sensor 3 of the third embodiment includes amplifiers A1 and A2 that receive temperature signals, and controls the output voltages of the variable power supplies 22 and 23 based on the output signals of the amplifiers A1 and A2, which is different from the gas sensor 1 of the first embodiment in this regard. Other basic structures are the same as those of the gas sensor 1 of the first embodiment, so the same reference numerals are assigned to the same elements, and repeated descriptions are omitted.

[0052] Amplifiers A1 and A2 generate control signals by amplifying temperature signals, and supply the generated control signals to variable power supplies 22 and 23, respectively. The gains of amplifiers A1 and A2 are different from each other according to the target heating temperature. That is, amplifier A1 for controlling the voltage value of variable power supply 22 has a gain for heating heater resistor MH1 to 150°C using the control signal generated based on the temperature signal, and amplifier A2 for controlling the voltage value of variable power supply 23 has a gain for heating heater resistor MH2 to 300°C using the control signal generated based on the temperature signal.

[0053] As exemplified in this embodiment, the control of variable power supplies 22 and 23 does not need to be digital control using MPU 24, and can also be analog control using amplifiers A1 and A2. Thereby, the processing load on MPU 24 is reduced, and temperature errors caused by the conversion error of AD converter 21 do not occur.

[0054] Figure 7 It is a circuit diagram showing the structure of gas sensor 4 according to the fourth embodiment of the technology of the present disclosure.

[0055] As Figure 7 shown, gas sensor 4 of the fourth embodiment includes amplifiers A1 and A2 that receive temperature signals, and controls the current output from variable current sources 27 and 28 based on the output signals of amplifiers A1 and A2, which is different from gas sensor 2 of the second embodiment in this regard. Other basic structures are the same as those of gas sensor 2 of the second embodiment, so the same reference numerals are assigned to the same elements, and repeated descriptions are omitted.

[0056] Amplifiers A1 and A2 generate control signals by amplifying temperature signals, and supply the generated control signals to variable current sources 27 and 28, respectively. The gains of amplifiers A1 and A2 are different from each other according to the target heating temperature. That is, amplifier A1 for controlling the current value of variable current source 27 has a gain for heating heater resistor MH1 to 150°C using the control signal generated based on the temperature signal, and amplifier A2 for controlling the current value of variable current source 28 has a gain for heating heater resistor MH2 to 200°C using the control signal generated based on the temperature signal.

[0057] Above, the embodiments of the present disclosure have been described, but the present disclosure is not limited to the above embodiments, and various changes can be made without departing from the gist of the present disclosure, and these are of course included in the scope of the present disclosure.

[0058] For example, in the above-described embodiment, the case where the gas to be measured is CO2 gas has been described as an example, but the present invention is not limited thereto. In addition, the sensor unit used in the present invention does not necessarily have to be a heat conduction type sensor, and may be a sensor of other types such as a catalytic combustion type. As an example, in the case where the gas to be measured is CO gas, a catalytic combustion type sensor unit can be used.

[0059] The technology disclosed in the present disclosure includes, but is not limited to, the following structural examples.

[0060] The gas sensor according to the present disclosure includes: a first thermistor and a second thermistor connected in series; a first heater and a second heater for heating the first thermistor and the second thermistor, respectively; and a control circuit that, in a state where the first heater and the second heater are heated, generates an output signal representing the concentration of the gas to be measured based on a detection signal that appears at a connection point between the first thermistor and the second thermistor, and the control circuit controls the heating temperatures of the first heater and the second heater based on the resistance value of the first heater or the second heater. Thus, the heating temperatures of the first heater and the second heater can be controlled without using a dedicated temperature sensor unit.

[0061] In the above gas sensor, it may also be that the control circuit controls the heating temperatures of the first heater and the second heater based on the resistance value of one of the first heater and the second heater in a state where the first heater and the second heater are heated, and generates an output signal based on the detection signal that appears at the connection point in this state. Thus, the heating temperatures of the first heater and the second heater can be controlled without measuring the resistance value of the other of the first heater and the second heater.

[0062] In the above gas sensor, it may also be that the first heater and the second heater are made of a conductive material having a positive resistance temperature coefficient, the first heater is heated to a first temperature, the second heater is heated to a second temperature higher than the first temperature, and the control circuit controls the heating temperatures of the first heater and the second heater based on the resistance value of the second heater in a state where the second heater is heated. Thus, more accurate temperature measurement can be performed.

[0063] In the above gas sensor, it may also be that the control circuit includes a first variable power source for applying a voltage to the first heater and a second variable power source for applying a voltage to the second heater, and controls the output voltages of the first variable power source and the second variable power source based on the resistance value of the second heater in a state where the second heater is heated. Thus, the heating temperatures of the first heater and the second heater can be controlled by voltage control.

[0064] In the above-described gas sensor, alternatively, the control circuit may include a first variable current source that supplies current to the first heater and a second variable current source that supplies current to the second heater, and controls the output currents of the first variable current source and the second variable current source based on the resistance value of the second heater in a state where the second heater is heated. Thereby, the heating temperatures of the first heater and the second heater can be controlled by current control.

[0065] In the above-described gas sensor, alternatively, the control circuit may include a fixed resistor connected in series with the second heater, and controls the heating temperatures of the first heater and the second heater based on the temperature signal that appears at the connection point between the second heater and the fixed resistor. Thereby, the resistance value of the second heater can be easily measured.

[0066] In the above-described gas sensor, alternatively, the control circuit may further include: an AD converter that converts the temperature signal into a digital value; and an MPU that calculates the voltage value or current value applied to the first heater and the second heater based on the digital value. Thereby, the heating temperatures of the first heater and the second heater can be controlled by digital control.

[0067] In the above-described gas sensor, alternatively, the control circuit may further include: a first amplifier that generates a first control signal based on the temperature signal; and a second amplifier that generates a second control signal based on the temperature signal, wherein the voltage value or current value applied to the first heater is controlled by the output signal of the first amplifier, and the voltage value or current value applied to the second heater is controlled by the output signal of the second amplifier. Thereby, the heating temperatures of the first heater and the second heater can be controlled by analog control.

Claims

1. A gas sensor, wherein, Comprising: A first thermistor and a second thermistor connected in series; A first heater and a second heater for heating the first thermistor and the second thermistor respectively; And A control circuit, which generates an output signal representing the concentration of the gas to be measured based on the detection signal appearing at the connection point between the first thermistor and the second thermistor in a state where the first heater and the second heater are heated. The control circuit controls the heating temperature of the first heater and the second heater based on the resistance value of the first heater or the second heater.

2. The gas sensor according to claim 1, wherein, The control circuit controls the heating temperature of the first heater and the second heater based on the resistance value of one of the first heater and the second heater in a state where the first heater and the second heater are heated, and generates the output signal based on the detection signal appearing at the connection point in this state.

3. The gas sensor according to claim 2, wherein, The first heater and the second heater are made of a conductive material having a positive temperature coefficient of resistance. The first heater is heated to a first temperature. The second heater is heated to a second temperature higher than the first temperature. The control circuit controls the heating temperature of the first heater and the second heater based on the resistance value of the second heater in a state where the second heater is heated.

4. The gas sensor according to claim 3, wherein, The control circuit includes a first variable power supply for applying voltage to the first heater and a second variable power supply for applying voltage to the second heater, and controls the output voltages of the first variable power supply and the second variable power supply based on the resistance value of the second heater in a state where the second heater is heated.

5. The gas sensor according to claim 3, wherein, The control circuit includes a first variable current source for supplying current to the first heater and a second variable current source for supplying current to the second heater, and controls the output currents of the first variable current source and the second variable current source based on the resistance value of the second heater in a state where the second heater is heated.

6. The gas sensor according to any one of claims 3 to 5, wherein, The control circuit includes a fixed resistor connected in series with the second heater, and controls the heating temperature of the first heater and the second heater based on the temperature signal appearing at the connection point between the second heater and the fixed resistor.

7. The gas sensor according to claim 6, wherein, The control circuit further includes: an AD converter for converting the temperature signal into a digital value; and an MPU for calculating the voltage value or current value applied to the first heater and the second heater based on the digital value.

8. The gas sensor according to claim 6, wherein, The control circuit further includes: a first amplifier that generates a first control signal based on the temperature signal; and a second amplifier that generates a second control signal based on the temperature signal, The voltage value or current value applied to the first heater is controlled by the output signal of the first amplifier, The voltage value or current value applied to the second heater is controlled by the output signal of the second amplifier.

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