Gas sensor

Abstract

According to the invention, measurement errors caused by annual changes of a thermistor are reduced. A gas sensor (1) is provided with: a detection circuit (10) including thermistors (Rd1, Rd2); and a control circuit (20) that controls the connection relationship of the detection circuit (10) and the heater resistors (MH1, MH2). During a gas measurement operation, the control circuit (20) connects the thermistors (Rd1, Rd2) in series and heats the heater resistors (MH1, MH2), thereby generating 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), and during a resistance measurement operation, generating 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) measures the resistance value of the thermistor (Rd1) in a state where the series connection of the thermistors (Rd1, Rd2) is released. The control circuit (20) corrects the value of the output signal (OUT) on the basis of the resistance value of the thermistor (Rd1). As a result, measurement errors caused by annual changes in the thermistor are corrected.

Description

Technical Field

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

[0002] In Patent Document 1, a gas sensor is disclosed 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 two thermistors connected in series. In the gas sensor described in Patent Document 1, after obtaining a detection signal by heating the thermistor constituting the detection element to 150°C and heating the thermistor constituting the reference element to 300°C, the difference in the thermal history of the two thermistors is eliminated by heating the thermistor constituting the detection element to 300°C and heating the thermistor constituting the reference element to 150°C.

[0003] Prior Art Documents

[0004] Patent Documents

[0005] Patent Document 1: International Publication No. WO2020 / 031517 Summary of the Invention

[0006] Technical Problem to be Solved by the Invention

[0007] However, in the gas sensor described in Patent Document 1, the temperature characteristics of the thermistor sometimes change over time.

[0008] In the present disclosure, a gas sensor is described that reduces the measurement error caused by the change over time of the thermistor.

[0009] Means for Solving the Technical Problem

[0010] The gas sensor of the present invention includes: a detection circuit including a first thermistor and a second thermistor; a first heater and a second heater that heat the first thermistor and the second thermistor respectively; and a control circuit that controls the connection relationship of the detection circuit and the first heater and the second heater. During a gas measurement operation, the control circuit connects the first thermistor and the second thermistor in series and heats the first heater and the second heater, thereby generating 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. During a resistance measurement operation, the control circuit measures the resistance value of the first thermistor in a state where the series connection of the first thermistor and the second thermistor is released, and corrects the value of the output signal based on the resistance value of the first thermistor measured during the resistance measurement operation.

[0011] Effects of the Invention

[0012] According to the present disclosure, it is possible to provide a gas sensor that reduces measurement errors caused by secular changes in a thermistor. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a circuit diagram showing the structure of a gas sensor 1 according to a first embodiment of the technology based on the present disclosure.

[0014] Figure 2 It is a circuit diagram of a resistance measurement circuit 11.

[0015] Figure 3 It is a timing chart for explaining the operation of the gas sensor 1.

[0016] Figure 4 It is a graph showing the temperature characteristics of thermistors Rd1 and Rd2.

[0017] Figure 5 It is a graph showing the relationship between the temperature of thermistors Rd1 and Rd2 and the sensitivity to CO2 gas.

[0018] Figure 6 It is a circuit diagram showing the structure of a gas sensor 2 according to a second embodiment of the technology based on the present disclosure.

[0019] Figure 7 It is a timing chart for explaining the operation of the gas sensor 2.

[0020] Figure 8 It is a circuit diagram showing the structure of a gas sensor 3 according to a third embodiment of the technology based on the present disclosure.

[0021] Figure 9 It is a circuit diagram showing the structure of a gas sensor 4 according to a fourth embodiment of the technology based on the present disclosure.

[0022] SYMBOL DESCRIPTION

[0023] 1 to 4 Gas sensors

[0024] 10 Detection circuit

[0025] 11, 12 Resistance measurement circuits

[0026] 11a One end

[0027] 11b The other end

[0028] 13 Constant current source

[0029] 14 Voltmeter

[0030] 15 Constant voltage source

[0031] 16 Ammeter

[0032] 20 Control circuit

[0033] 21 AD converter

[0034] 22, 23 DA converters

[0035] 24 MPU

[0036] 25 Power supply

[0037] 26 Multiplexer

[0038] Heater resistors MH1, MH2

[0039] Fixed resistors R1 to R3

[0040] Thermistors Rd1 to Rd3

[0041] Switches SW1 to SW3, SW11, SW12

[0042] a Common node

[0043] b, c Selection nodes Detailed implementation manners

[0044] In the following, the implementation manners of the technology of the present disclosure will be described in detail with reference to the drawings.

[0045] Figure 1 It is a circuit diagram showing the structure of the gas sensor 1 according to the first implementation manner of the technology of the present disclosure.

[0046] As Figure 1 shown, the gas sensor 1 according to the first implementation manner includes: a detection circuit 10 including thermistors Rd1, Rd2 and resistance measurement circuits 11, 12; heaters MH1, MH2 for heating the thermistors Rd1, Rd2 respectively; and a control circuit 20 for controlling the detection circuit 10 and the heaters MH1, MH2. The gas sensor 1 according to the present implementation manner is a heat conduction type gas sensor for detecting the concentration of CO2 gas in the atmosphere, but is not particularly limited.

[0047] The detection circuit 10 includes thermistors Rd1, Rd2, resistance measurement circuits 11 and 12, and switches SW1 to SW3. The thermistors Rd1, Rd2 are detection elements made of materials having a negative resistance temperature coefficient such as composite metal oxides, amorphous silicon, polycrystalline silicon, germanium, etc., for example. Both the thermistors Rd1, 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.

[0048] As Figure 1As shown, the switch SW1 is connected between the power supply 25 that supplies the power supply potential VDDS and the thermistor Rd1. The switches SW2 and SW3 are connected between the thermistor Rd1 and the thermistor Rd2. Thus, when the switches SW1 to SW3 are turned on, the thermistor Rd1 and the thermistor Rd2 are connected in series between the power supply 25 and the ground. In this state, the potential that appears between the switch SW2 and the switch SW3, that is, the detection signal that appears at the connection point of the thermistor Rd1 and the thermistor Rd2, is supplied to the control circuit 20. In contrast, when the switches SW1 to SW3 are turned off, the series connection of the thermistor Rd1 and the thermistor Rd2 is released, and the two are disconnected from each other. The resistance measurement circuits 11 and 12 are circuits that connect the resistance values of the thermistors Rd1 and Rd2, respectively, in a state where the switches SW1 to SW3 are turned off.

[0049] As Figure 2 shown in (a) of, the resistance measurement circuit 11 may have a structure in which a constant current source 13 and a voltmeter 14 are connected in parallel between one end 11a and the other end 11b. Accordingly, in a state where the thermistor Rd1 is connected between one end 11a and the other end 11b, if a constant current flows from the constant current source 13 to the thermistor Rd1, the voltage generated between one end 11a and the other end 11b is determined by the resistance value of the thermistor Rd1. This voltage is measured by the voltmeter 14 and supplied to the control circuit 20. Thus, the control circuit 20 can obtain the resistance value of the thermistor Rd1 measured directly.

[0050] Alternatively, as Figure 2 shown in (b) of, it may have a structure in which a constant voltage source 15 and an ammeter 16 are connected in series between one end 11a and the other end 11b. Accordingly, in a state where the thermistor Rd1 is connected between one end 11a and the other end 11b, if a predetermined voltage is applied from the constant voltage source 15 to the thermistor Rd1, the current flowing between one end 11a and the other end 11b is determined by the resistance value of the thermistor Rd1. This current is measured by the ammeter 16 and supplied to the control circuit 20. Thus, the control circuit 20 can obtain the resistance value of the thermistor Rd1 measured directly.

[0051] The structure of the resistance measurement circuit 12 is the same, and the control circuit 20 can obtain the resistance value of the thermistor Rd2 measured directly.

[0052] The control circuit 20 includes an AD converter (ADC) 21, DA converters (DAC) 22, 23, an MPU 24, a power supply 25, and a multiplexer 26. Under the control of the MPU 24, the multiplexer 26 supplies either the detection signal that appears at the connection point of the thermistors Rd1 and Rd2 or the resistance measurement signals output from the resistance measurement circuits 11, 12 to the AD converter 21. Multiple AD converters 21 may be provided instead of using the multiplexer 26. The MPU 24 controls the connection relationship of the detection circuit 10 by controlling the switches SW1 to SW3. The MPU 24 turns the switches SW1 to SW3 off during the resistance measurement operation. The AD converter 21 sequentially performs AD conversion on the resistance measurement signals supplied from the resistance measurement circuits 11, 12 in this state and supplies the values to the MPU 24. The MPU 24 calculates the resistance values of the thermistors Rd1, Rd2 based on the resistance measurement signals obtained by AD conversion. On the other hand, the MPU 24 turns the switches SW1 to SW3 on during the gas measurement operation. The AD converter 21 performs AD conversion on the detection signal that appears at the connection point of the thermistors Rd1 and Rd2 in this state and provides the value to the MPU 24. The MPU 24 calculates the output signal OUT indicating the concentration of CO2 gas based on the detection signal obtained by AD conversion. In the calculation of the output signal OUT, the value of the output signal OUT is corrected based on the resistance values of the thermistors Rd1, Rd2 calculated during the resistance measurement operation.

[0053] The DA converters 22, 23 apply a prescribed voltage to the heater resistors MH1, MH2 by performing DA conversion on the digital values supplied from the MPU 24. That is, the heating temperature of the heater resistors MH1, MH2 is controlled by the MPU 24.

[0054] Next, the operation of the gas sensor 1 of the present embodiment will be described.

[0055] Figure 3 is a timing chart for explaining the operation of the gas sensor 1 of the present embodiment.

[0056] As Figure 3 shown, the gas sensor 1 of the present embodiment performs a gas measurement operation during the period T1 and a virtual heating operation during the period T2. The gas measurement operation and the virtual heating operation are performed alternately. Also, at the timing immediately before the period T1 when the gas measurement operation is to be performed, i.e., at time t1, a resistance measurement operation is performed. During the resistance measurement operation, the heating of the thermistors Rd1, Rd2 by the heater resistors MH1, MH2 is stopped. Therefore, the resistance measurement operation performed at time t1 is executed by the thermistors Rd1, Rd2 at the ambient temperature.

[0057] In the present embodiment, a temperature signal TP indicating the current ambient temperature is supplied to the MPU 24. Thereby, the MPU 24 can acquire the current ambient temperature and the resistance values of the thermistors Rd1 and Rd2 at the current ambient temperature, and based on this information, can calculate the resistance values of the thermistors Rd1 and Rd2 at a specified temperature (e.g., 25°C) serving as a reference. Here, the design values of the resistances of the thermistors Rd1 and Rd2 at the specified temperature (e.g., 25°C) serving as a reference are stored in the MPU 24, and the MPU 24 corrects the value of the output signal based on the resistance values of the thermistors by comparing the design values of the resistances of the thermistors Rd1 and Rd2 with the actual resistance values.

[0058] In the gas measurement operation performed during the period T1, 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. As Figure 4 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 4 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.

[0059] Figure 5 is a graph showing the relationship between the temperature of the thermistors Rd1 and Rd2 and the sensitivity to CO2 gas. As Figure 5 shown, the sensitivities of the thermistors Rd1 and Rd2 to CO2 gas vary greatly depending on the temperature, and in the temperature range of 40°C or lower or 300°C or higher, the sensitivities of the thermistors Rd1 and Rd2 to CO2 gas are substantially zero. In contrast, the sensitivities of the thermistors Rd1 and Rd2 to CO2 gas become maximum in a state of approximately 150°C.

[0060] Therefore, when there is CO2 gas in the measurement atmosphere while the thermistor Rd1, which is a detection element, is heated to 150°C, 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, even when there is CO2 gas in the measurement atmosphere while the thermistor Rd2, which is a reference element, is heated to 300°C, 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 heated to 300°C 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 heated to 300°C caused by the concentration of CO2 gas can also be almost negligible.

[0061] Accordingly, the level of the detection signal that appears at the connection point between the thermistor Rd1 and the thermistor Rd2 changes according to the concentration of CO2 gas in the measurement atmosphere. The detection signal is supplied to the MPU24 via the AD converter 21, and the MPU24 generates an output signal OUT representing the concentration of CO2 gas based on this. In the calculation of the output signal OUT, correction is performed based on the result of the resistance measurement operation carried out at time t1.

[0062] As described above, the resistance measurement operation carried out at time t1 is performed at an ambient temperature where the sensitivity of the thermistors Rd1 and Rd2 to CO2 gas is approximately zero. Therefore, regardless of the concentration of CO2 gas in the environment during the resistance measurement operation, the resistance values of the thermistors Rd1 and Rd2 can be accurately measured. Accordingly, it is possible to calculate the resistance value of the thermistor Rd1 heated to 150°C and the resistance value of the thermistor Rd2 heated to 300°C when the concentration of CO2 gas is the same as that in normal atmosphere (about 400 ppm). Thus, it is possible to calculate the output signal OUT representing the concentration of CO2 gas using the detection signal that actually appears at the connection point between the thermistor Rd1 and the thermistor Rd2, the normal resistance value of the thermistor Rd1 heated to 150°C, and the normal resistance value of the thermistor Rd2 heated to 300°C.

[0063] During the virtual heating operation carried out during period T2, under the control of the MPU24, the heater resistance MH1 is heated to 300°C and the heater resistance MH1 is heated to 150°C. Thereby, the difference in the thermal history between the thermistor Rd1 and the thermistor Rd2 during the gas measurement operation carried out during period T1 is canceled out.

[0064] Thus, since the gas sensor 1 of the present embodiment actually measures the resistance values of the thermistors Rd1 and Rd2 using the resistance measurement circuits 11 and 12, even when the thermistors Rd1 and Rd2 have undergone secular changes, the value of the output signal OUT can be corrected correctly. Moreover, the measurement of the resistance values of the thermistors Rd1 and Rd2 using the resistance measurement circuits 11 and 12 is performed in a state where the heating by the heater resistors MH1 and MH2 is stopped, so measurement errors caused by the concentration of CO2 gas present in the atmosphere do not occur. In addition, by performing the resistance measurement operation just before the period T1, the influence of afterheat can be excluded, and the resistance values of the thermistors Rd1 and Rd2 can be measured at a more accurate ambient temperature.

[0065] However, when the actual ambient temperature deviates significantly from the specified temperature (e.g., 25°C) used as a reference, there is a concern that the resistance measurement operation may be affected by CO2 gas. Therefore, when the ambient temperature is outside the specified temperature range, for example, when it exceeds 40°C, the MPU 24 can also invalidate the resistance measurement operation. In this case, the resistance measurement operation itself can be skipped, or although the resistance measurement operation is executed, the arithmetic operation can be skipped. Alternatively, it can also be that although the resistance measurement operation and the arithmetic operation are executed, the values obtained thereby are ignored.

[0066] In addition, in the present embodiment, since the gas measurement operation is performed during the period T1 and the dummy heating operation is performed during the period T2, the secular changes of the thermistors Rd1 and Rd2 are substantially the same. Considering this, it is not necessary to actually measure the resistance values of both the thermistors Rd1 and Rd2. It is also possible to actually measure only the resistance value of one of the thermistors Rd1 and Rd2 and calculate the resistance value of the other thermistor Rd2 based on the result. For example, it is also possible to calculate both the resistance value of the thermistor Rd1 heated to 150°C and the resistance value of the thermistor Rd2 heated to 300°C by measuring the resistance value of the thermistor Rd1 at room temperature.

[0067] In addition, in the present embodiment, the heating by the heater resistors MH1 and MH2 is stopped during the resistance measurement operation, but as long as the sensitivity of the thermistors Rd1 and Rd2 to CO2 gas is sufficiently low, the heating by the heater resistors MH1 and MH2 can also be performed. As an example, the thermistors Rd1 and Rd2 can also be heated to 300°C by the heater resistors MH1 and MH2 during the resistance measurement operation.

[0068] Figure 6 It is a circuit diagram showing the structure of the gas sensor 2 according to the second embodiment of the technology based on the present disclosure.

[0069] As Figure 6As shown, in the gas sensor 2 of the second embodiment, the detection circuit 10 includes a thermistor Rd3 and a fixed resistor R3, which is different from the gas sensor 1 of the first embodiment. The other basic structures are the same as those of the gas sensor 1 based on the first embodiment. Therefore, the same reference numerals are used for the same elements, and repeated descriptions are omitted.

[0070] The thermistor Rd3 and the fixed resistor R3 are connected in series between the power supply 25 and the ground, and a temperature signal TP appears at their connection point. The temperature signal TP is supplied to the AD converter 21 via the multiplexer 26. The AD converter 21 performs AD conversion on the temperature signal TP and supplies its value to the MPU 24.

[0071] Figure 7 is a timing chart for explaining the operation of the gas sensor 2 of the present embodiment.

[0072] As Figure 7 shown, the gas sensor 2 based on the present embodiment acquires the temperature signal TP at the time t2, which is the timing just before the period T1, and performs the resistance measurement operation at the time t1. The order of the time t1 and the time t2 is not limited, but it is preferable that the time difference between the two is small. Thereby, the ambient temperature at the time of the resistance measurement operation can be measured more accurately.

[0073] As exemplified in the second embodiment, the temperature signal TP may also be generated by the detection circuit 10 itself.

[0074] Figure 8 is a circuit diagram showing the structure of the gas sensor 3 of the third embodiment of the technology of the present invention.

[0075] As Figure 8 shown, in the gas sensor 3 of the third embodiment, the detection circuit 10 includes fixed resistors R1, R2 and switches SW11, SW12, which is different from the gas sensor 1 of the first embodiment. The other basic structures are the same as those of the gas sensor 1 based on the first embodiment. Therefore, the same reference numerals are used for the same elements, and repeated descriptions are omitted.

[0076] Both switches SW11 and SW12 have a common node a and two selection nodes b and c, and one of the selection nodes b and c is connected to the common node a. Both switches SW11 and SW12 are configured to select the selection node b during the gas measurement operation and the virtual heating operation, and select the selection node c during the resistance measurement operation.

[0077] As Figure 8As shown, the common node a of switch SW11 is connected to one end of the thermistor Rd1, the selection node b of switch SW11 is connected to the selection node b of switch SW12, and the selection node c of switch SW11 is connected to one end of the fixed resistor R1. The other end of the thermistor Rd1 is connected to the power supply 25 that supplies the power supply potential VDDS, and the other end of the fixed resistor R1 is connected to the wiring that is supplied with the ground potential GND. In addition, the common node a of switch SW12 is connected to one end of the thermistor Rd2, the selection node b of switch SW12 is connected to the selection node b of switch SW11, and the selection node c of switch SW12 is connected to one end of the fixed resistor R2. The other end of the fixed resistor R2 is connected to the power supply 25 that supplies the power supply potential VDDS, and the other end of the thermistor Rd2 is connected to the wiring that is supplied with the ground potential GND.

[0078] Thus, during the gas measurement operation and the virtual heating operation when the selection node b is selected, the thermistors Rd1 and Rd2 are connected in series between the power supply 25 and the ground. On the other hand, during the resistance measurement operation when the selection node c is selected, the thermistor Rd1 and the fixed resistor R1 are connected in series between the power supply 25 and the ground, and the fixed resistor R2 and the thermistor Rd2 are connected in series between the power supply 25 and the ground. As a result, during the resistance measurement operation, the resistance value of the thermistor Rd1 can be calculated based on the potential that appears at the connection point between the thermistor Rd1 and the fixed resistor R1, and the resistance value of the thermistor Rd2 can be calculated based on the potential that appears at the connection point between the fixed resistor R2 and the thermistor Rd2.

[0079] Figure 9 It is a circuit diagram showing the structure of the gas sensor 4 according to the fourth embodiment based on the technology of the present disclosure.

[0080] As Figure 9 shown, in the gas sensor 4 according to the fourth embodiment, the detection circuit 10 includes a thermistor Rd3 and a fixed resistor R3, which is different from the gas sensor 3 of the third embodiment in this regard. The other basic structure is the same as that of the gas sensor 3 of the third embodiment, so the same reference numerals are assigned to the same elements, and repeated descriptions are omitted.

[0081] The thermistor Rd3 and the fixed resistor R3 are connected in series between the power supply 25 and the ground, and a temperature signal TP appears at their connection point. The temperature signal TP is supplied to the AD converter 21 via the multiplexer 26. The AD converter 21 performs AD conversion on the temperature signal TP and supplies its value to the MPU 24.

[0082] The operation of the gas sensor 4 of the present embodiment is as Figure 7As shown, the temperature signal TP is acquired at time t2 which is the timing just before period T1, and the resistance measurement operation is performed at time t1.

[0083] As described above, embodiments of the present disclosure have been described. However, the present disclosure is not limited to the above-described 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.

[0084] For example, in the above 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 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.

[0085] The technology of the present disclosure includes but is not limited to the following structural examples.

[0086] The gas sensor of the present disclosure includes: a detection circuit including a first thermistor and a second thermistor; a first heater and a second heater that heat the first thermistor and the second thermistor respectively; and a control circuit that controls the connection relationship of the detection circuit and the first heater and the second heater. During the gas measurement operation, the control circuit connects the first thermistor and the second thermistor in series and heats the first heater and the second heater, thereby generating an output signal representing the concentration of the gas to be measured based on the detection signal that appears at the connection point of the first thermistor and the second thermistor; during the resistance measurement operation, the control circuit measures the resistance value of the first thermistor in a state where the series connection of the first thermistor and the second thermistor is released, and the control circuit corrects the value of the output signal based on the resistance value of the first thermistor measured during the resistance measurement operation. Thus, since the resistance value of the first thermistor is actually measured, it is possible to correct the measurement error caused by the secular change of the first thermistor and the second thermistor.

[0087] In the above gas sensor, it may be that the control circuit stops heating the first heater during the resistance measurement operation. Thus, it is possible to measure the resistance value of the first thermistor at the ambient temperature.

[0088] In the above gas sensor, it may be that the control circuit calculates the resistance value of the first thermistor at a specified temperature based on the resistance value of the first thermistor and the ambient temperature. Thus, it is possible to calculate the resistance value of the first thermistor at the specified temperature more accurately.

[0089] In the above-described gas sensor, alternatively, when the ambient temperature is outside a specified temperature range, the control circuit invalidates the measurement of the resistance value of the first thermistor. Thereby, it is possible to avoid correcting the output signal based on the resistance value of the first thermistor calculated in an environment with a large measurement error.

[0090] In the above-described gas sensor, alternatively, during the resistance measurement operation, the control circuit further measures the resistance value of the second thermistor in a state where the series connection of the first thermistor and the second thermistor is released, and corrects the value of the output signal based on the resistance values of the first thermistor and the second thermistor. Thereby, since the resistance values of the first thermistor and the second thermistor are actually measured, it is possible to more accurately correct the measurement error caused by the secular change of the first thermistor and the second thermistor.

[0091] In the above-described gas sensor, alternatively, the control circuit stops heating the second heater during the resistance measurement operation. Thereby, it is possible to measure the resistance value of the second thermistor at the ambient temperature.

[0092] In the above-described gas sensor, alternatively, the detection circuit further includes a switch connected between the first thermistor and the second thermistor, and the control circuit closes the switch during the gas measurement operation and opens the switch during the resistance measurement operation. Thereby, it is possible to actually measure the resistance value of the first thermistor using the resistance measurement circuit.

[0093] In the above-described gas sensor, alternatively, the detection circuit further includes a first fixed resistor, and during the resistance measurement operation, the control circuit connects the first thermistor and the first fixed resistor in series, and thereby measures the resistance value of the first thermistor based on the potential appearing at the connection point between the first thermistor and the first fixed resistor. Thereby, it is possible to actually measure the resistance value of the first thermistor without using a resistance measurement circuit.

[0094] In the above-described gas sensor, alternatively, the detection circuit further includes a second fixed resistor, and during the resistance measurement operation, the control circuit connects the second thermistor and the second fixed resistor in series, and thereby measures the resistance value of the second thermistor based on the potential appearing at the connection point between the second thermistor and the second fixed resistor. Thereby, it is possible to actually measure the resistance value of the second thermistor without using a resistance measurement circuit.

Claims

1. A gas sensor, wherein, It comprises: A detection circuit including a first thermistor and a second thermistor; A first heater and a second heater for heating the first thermistor and the second thermistor respectively; and A control circuit for controlling the connection relationship of the detection circuit and the first heater and the second heater, During the gas measurement operation, the control circuit connects the first thermistor and the second thermistor in series, and heats the first heater and the second heater. Thus, an output signal representing the concentration of the gas to be measured is generated based on the detection signal appearing at the connection point between the first thermistor and the second thermistor. During the resistance measurement operation, the control circuit measures the resistance value of the first thermistor in a state where the series connection of the first thermistor and the second thermistor is released. The control circuit corrects the value of the output signal based on the resistance value of the first thermistor measured during the resistance measurement operation.

2. The gas sensor according to claim 1, wherein, The control circuit stops heating the first heater during the resistance measurement operation.

3. The gas sensor according to claim 2, wherein, The control circuit calculates the resistance value of the first thermistor at a specified temperature based on the resistance value of the first thermistor and the ambient temperature.

4. The gas sensor according to claim 3, wherein, When the ambient temperature is outside a specified temperature range, the control circuit invalidates the measurement of the resistance value of the first thermistor.

5. The gas sensor according to claim 1, wherein, During the resistance measurement operation, the control circuit further measures the resistance value of the second thermistor in a state where the series connection of the first thermistor and the second thermistor is released, and corrects the value of the output signal based on the resistance values of the first thermistor and the second thermistor.

6. The gas sensor according to claim 5, wherein, The control circuit stops heating the second heater during the resistance measurement operation.

7. The gas sensor according to any one of claims 1 to 6, wherein, The detection circuit further includes a switch connected between the first thermistor and the second thermistor, The control circuit closes the switch during the gas measurement operation and opens the switch during the resistance measurement operation.

8. The gas sensor according to any one of claims 1 to 6, wherein, The detection circuit further includes a first fixed resistor, During the resistance measurement operation, the control circuit connects the first thermistor and the first fixed resistor in series, and thus measures the resistance value of the first thermistor based on the potential appearing at the connection point between the first thermistor and the first fixed resistor.

9. The gas sensor according to claim 8, wherein, The detection circuit further includes a second fixed resistor, During the resistance measurement operation, the control circuit connects the second thermistor and the second fixed resistor in series, and thereby measures the resistance value of the second thermistor based on the potential that appears at the connection point between the second thermistor and the second fixed resistor.

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