Gas sensor

Abstract

The purpose of the present invention is to reduce power consumption in a gas sensor of the type in which a thermistor is heated during measurement. A gas sensor (1) is provided with thermistors (Rd1, Rd2) connected in series, heater resistors (MH1, MH2) for heating the thermistors (Rd1, Rd2), respectively, and a control circuit (20) for controlling the heater resistors (MH1, MH2). The control circuit (20) heats the heater resistor (MH2) to a temperature higher than that of the heater resistor (MH1), in this state, 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), and sets the heating time of the heater resistor (MH1) to be shorter than the heating time of the heater resistor (MH2). Thus, power consumption is reduced.

Description

Technical Field

[0001] The present invention 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, a detection signal is obtained by heating the thermistor constituting the detection element to 150°C and heating the thermistor constituting the reference element to 300°C.

[0003] Prior Art Documents

[0004] Patent Documents

[0005] Patent Document 1: International Publication WO2020 / 031517 Summary of the Invention

[0006] Technical Problem to be Solved by the Invention

[0007] For a gas sensor of the type that heats a thermistor during measurement, as in the gas sensor described in Patent Document 1, a reduction in power consumption is required.

[0008] In the present invention, a technique for reducing power consumption in a gas sensor of the type that heats a thermistor during measurement is described.

[0009] Technical Means for Solving the Technical Problem

[0010] The gas sensor of the present invention 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 for controlling the first heater and the second heater. The control circuit heats the second heater to a higher temperature than the first heater during a first period, and based on a detection signal that appears at the connection point of the first thermistor and the second thermistor in this state, generates an output signal representing the concentration of the gas to be measured, and makes the heating time of the first heater shorter than the heating time of the second heater.

[0011] Effects of the Invention

[0012] According to the present invention, a technique for reducing power consumption in a gas sensor of the type that heats a thermistor during measurement is provided. 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 technique of the present invention.

[0014] Figure 2 It is a timing chart for explaining the operation of the gas sensor 1.

[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 graph for explaining the time change of the heating temperature of the thermistors Rd1 and Rd2 during period A.

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

[0019] Hereinafter, embodiments of the technology according to the present invention will be described in detail with reference to the accompanying drawings.

[0020] Figure 1 It is a circuit diagram showing the structure of the gas sensor 1 according to the first embodiment of the technology of the present invention.

[0021] As Figure 1 shown, the gas sensor 1 of the first embodiment includes thermistors Rd1 and Rd2, heater resistors MH1 and MH2 for heating the thermistors Rd1 and Rd2 respectively, and a control circuit 20 for controlling the heater resistors MH1 and MH2. Although not particularly limited, 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.

[0022] The thermistors Rd1 and Rd2 are detection elements made of materials having a negative temperature coefficient of resistance, such as composite metal oxides, amorphous silicon, polycrystalline silicon, and germanium. Both the thermistors Rd1 and Rd2 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 a detection element, and the thermistor Rd2 constitutes a reference element. The thermistor Rd1 and the thermistor Rd2 are connected in series between the power supply 25 supplying the power supply potential VDDS and the ground, and the detection signal appearing at the connection point between the two is supplied to the control circuit 20.

[0023] The control circuit 20 includes an AD converter (ADC) 21, DA converters (DAC) 22, 23, an MPU 24, and a power supply 25. The AD converter 21 performs AD conversion on the detection signal that appears at the connection point of the thermistors Rd1 and Rd2, and supplies the resulting digital value to the MPU 24. The MPU 24 generates an output signal OUT representing the concentration of CO2 gas based on the detection signal obtained by AD conversion. The DA converters 22, 23 apply a prescribed voltage to the heater resistors MH1, MH2 by performing DA conversion on the digital value supplied from the MPU 24. That is, the heating temperature of the heater resistors MH1, MH2 is controlled by the MPU 24.

[0024] Next, the operation of the gas sensor 1 of the present embodiment will be described.

[0025] Figure 2 is a timing chart for explaining the operation of the gas sensor 1 of the present embodiment.

[0026] As Figure 2 shown, the gas sensor 1 of the present embodiment performs a gas measurement operation during period A and a virtual heating operation during period B. The gas measurement operation and the virtual heating operation are performed alternately.

[0027] During the gas measurement operation performed during period A, 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. Here, since the heater resistor MH1 and the thermistor Rd1 are arranged extremely close to each other, the temperature of the heater resistor MH1 can be regarded as being substantially the same as the temperature of the thermistor Rd1. Similarly, since the heater resistor MH2 and the thermistor Rd2 are arranged extremely close to each other, the temperature of the heater resistor MH2 can be regarded as being substantially the same as the temperature of the thermistor Rd2.

[0028] As Figure 3 shown, the temperature characteristics of the thermistors Rd1, 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 Figure 3 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.

[0029] Figure 4 is a graph showing the relationship between the temperature of the thermistors Rd1, 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.

[0030] 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 when heated to 300°C is sufficiently small compared to the change in the resistance value of the thermistor Rd1 caused by the concentration of CO2 gas when heated to 150°C. The change in the resistance value of the thermistor Rd2 caused by the concentration of CO2 gas when heated to 300°C can also be almost negligible.

[0031] Thus, 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 accordingly.

[0032] Here, in the gas measurement operation performed during period A, the heating start timing of the heater resistor MH2 is time t11, the heating start timing of the heater resistor MH1 is time t12, and the heating end timing of the heater resistors MH1 and MH2 is time t13. Thus, the heating time of the heater resistor MH1 is the period T1 from time t12 to time t13, and the heating time of the heater resistor MH2 is the period T2 from time t11 to time t13.

[0033] Figure 5 It is a graph for explaining the time change of the heating temperatures of the thermistors Rd1 and Rd2 during period A.

[0034] As Figure 5 shown, when the heating of the heater resistor MH2 starts at time t11, the temperature of the thermistor Rd2 rises. However, it takes a prescribed time to reach the target value of 300°C. In Figure 5In the example shown, at time t0, the temperature of the thermistor Rd2 reaches 300°C, which is the target value. Additionally, when the heating of the heater resistor MH1 starts at a time t12 that is later than time t11, the temperature of the thermistor Rd1 rises. However, it takes a specified time to reach 150°C, which is the target value. Here, since the heating temperature of the thermistor Rd1 is lower than that of the thermistor Rd2, the time it takes for the temperature of the thermistor Rd1 to reach 150°C, which is the target value, is shorter than the time it takes for the temperature of the thermistor Rd2 to reach 300°C, which is the target value. In Figure 5 the example shown, at time t0, the temperature of the thermistor Rd1 reaches 150°C, which is the target value. Therefore, both the thermistors Rd1 and Rd2 are heated to the target temperature after time t0.

[0035] After that, at time t13, the heating of the heater resistors MH1 and MH2 ends simultaneously. Therefore, both the thermistors Rd1 and Rd2 are correctly heated to the target temperature during the period from time t0 to time t13, and the detection signals obtained during this period are sampled by the MPU24.

[0036] In this way, during period A, the heater resistor MH2, which is to be heated to a higher temperature, starts heating earlier than the heater resistor MH1. Therefore, it is possible to make the times (t0) at which both the thermistors Rd1 and Rd2 are heated to the target temperature approximately the same. As a result, compared with the case where the heating of the heater resistors MH1 and MH2 starts simultaneously, the heating time of the heater resistor MH1 can be shortened, and the power consumption can be reduced. Moreover, since the heating time of the heater resistor MH1 is shortened, the secular change of the heater resistor MH1 can also be suppressed. Additionally, it is not necessary for the times at which both the thermistors Rd1 and Rd2 are heated to the target temperature to be exactly the same. It is sufficient to narrow the timing difference at which both the thermistors Rd1 and Rd2 are heated to the target temperature by making the heating start timing of the heater resistor MH1 later than the heating start timing of the second heater. Also, it is not necessary for the heating end timing of the heater resistor MH1 to be exactly the same as the heating end timing of the heater resistor MH2, but by making the timings of the two the same, it is possible to prevent the generation of useless power consumption.

[0037] As Figure 3 shown, in the virtual heating operation performed during period B, under the control of the MPU24, the heater resistor MH1 is heated to 300°C, and the heater resistor MH1 is heated to 150°C.

[0038] During the virtual heating operation in period B, the heating start timing of the heater resistor MH1 is time t21, the heating start timing of the heater resistor MH2 is time t22, and the heating end timing of the heater resistors MH1 and MH2 is time t23. Accordingly, the heating time of the heater resistor MH2 is the period T1 from time t22 to time t23, and the heating time of the heater resistor MH1 is the period T2 from time t21 to time t23. Accordingly, the thermal history difference between the thermistors Rd1 and Rd2 during the gas measurement operation performed during the period T1 is canceled out.

[0039] In this way, during period B, since the heater resistor MH1, which starts heating earlier than the heater resistor MH2 and is to be heated to a higher temperature, can cause the times at which both the thermistors Rd1 and Rd2 are heated to the target temperature to be substantially the same. Accordingly, compared with the case where the heating of the heater resistors MH1 and MH2 starts simultaneously, the heating time of the heater resistor MH2 can be shortened, and power consumption can be reduced. Moreover, since the heating time of the heater resistor MH2 is shortened, secular changes in the heater resistor MH2 can also be suppressed. In addition, during period B, the detection signal is not sampled, and thus, there is no need to have a period during which both the thermistors Rd1 and Rd2 are heated to the target temperature. That is to say, the period of heating the heater resistor MH1 and the period of heating the heater resistor MH2 may not overlap. Even in this case, by making the heating period T1 of the heater resistor MH2 shorter than the heating period T1 of the heater resistor MH1, power consumption can be reduced.

[0040] In this way, in the gas sensor 1 according to the present embodiment, during period A when the gas measurement operation is performed, the heating period T1 of the heater resistor MH1 is made shorter than the heating period T2 of the heater resistor MH2, and during period B when the virtual heating operation is performed, the heating period T1 of the heater resistor MH2 is made shorter than the heating period T2 of the heater resistor MH1. Accordingly, power consumption can be reduced, and secular changes in the thermistors Rd1 and Rd2 can be suppressed.

[0041] Figure 6 It is a circuit diagram showing the structure of a gas sensor 2 according to a second embodiment of the technology of the present invention.

[0042] As Figure 6 shown, the difference between the gas sensor 2 according to the second embodiment and the gas sensor 1 according to the first embodiment is that a thermistor Rd3 and a fixed resistor R1 are added, and an AD converter 26 is included in the control circuit 20. Since the other basic structure is the same as that of the gas sensor 1 according to the first embodiment, the same reference numerals are given to the same elements, and redundant description is omitted.

[0043] The thermistor Rd3 and the fixed resistor R1 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 a signal representing the current ambient temperature and is supplied to the AD converter 26. The AD converter 26 performs AD conversion on the temperature signal TP and supplies its value to the MPU 24. In addition, a humidity signal H representing the current ambient humidity is supplied to the MPU 24.

[0044] In the present embodiment, based on the current ambient temperature represented by the temperature signal TP and the current ambient humidity represented by the humidity signal H, during the period A of performing the gas measurement operation and the period B of performing the virtual heating operation, the heating time of the heater resistors MH1 and MH2 is changed. Specifically, the lower the current ambient temperature represented by the temperature signal TP, the longer the time for the thermistors Rd1 and Rd2 to reach the target temperature. Therefore, by advancing the heating start timing of the heater resistors MH1 and MH2 or delaying the heating end timing of the heater resistors MH1 and MH2, the heating time of the heater resistors MH1 and MH2 is made longer. In addition, the higher the current ambient humidity represented by the humidity signal H, the higher the heat dissipation characteristics to the atmosphere, and the longer the time for the thermistors Rd1 and Rd2 to reach the target temperature. Therefore, by advancing the heating start timing of the heater resistors MH1 and MH2 or delaying the heating end timing of the heater resistors MH1 and MH2, the heating time of the heater resistors MH1 and MH2 can be made longer.

[0045] The embodiments of the present invention have been described above, but the present invention is not limited to the above embodiments, and various changes can be made without departing from the gist of the present invention, and these are of course included in the scope of the present invention.

[0046] 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 other types of sensors such as a catalytic combustion type can also be used. As an example, in the case where the gas to be measured is CO gas, a catalytic combustion type sensor unit can be used.

[0047] The technology of the present invention includes the following structural examples, but is not limited thereto.

[0048] The gas sensor of the present invention 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 for controlling the first heater and the second heater. The control circuit heats the second heater to a higher temperature than the first heater during a first period, and based on a detection signal appearing at the connection point of the first thermistor and the second thermistor in this state, generates an output signal representing the concentration of the gas to be measured, and makes the heating time of the first heater shorter than that of the second heater. Thereby, power consumption can be reduced, and the secular change of the first thermistor can be suppressed.

[0049] In the above gas sensor, it may also be that the control circuit makes the heating start timing of the first heater later than that of the second heater during the first period. Thereby, the time difference between the first thermistor and the second thermistor being heated to the target temperature can be reduced.

[0050] In the above gas sensor, it may also be that the control circuit makes the heating end timing of the first heater coincide with the heating end timing of the second heater during the first period. Thereby, power consumption can be further reduced.

[0051] In the above gas sensor, it may also be that the control circuit changes the heating times of the first heater and the second heater based on the ambient temperature or ambient humidity during the first period. Thereby, the heating times can be optimized according to the ambient temperature or ambient humidity.

[0052] In the above gas sensor, it may also be that the control circuit heats the first heater to a higher temperature than the second heater during a second period, and makes the heating time of the second heater shorter than that of the first heater. Thereby, power consumption can be reduced, the secular change of the second thermistor can be suppressed, and the difference in the thermal history between the first thermistor and the second thermistor can be offset.

[0053] In the above gas sensor, it may also be that the control circuit makes the heating start timing of the second heater later than that of the first heater during the second period. Thereby, the time difference between the first thermistor and the second thermistor being heated to the target temperature can be reduced.

[0054] In the above gas sensor, it may also be that the control circuit makes the heating end timing of the first heater coincide with the heating end timing of the second heater during the second period. Thereby, power consumption can be further reduced.

[0055] In the above gas sensor, it is also possible that the control circuit changes the heating time of the first heater and the second heater based on the ambient temperature or the ambient humidity during the second period. Thereby, the heating time can be optimized according to the ambient temperature or the ambient humidity.

[0056] Explanation of symbols

[0057] 1, 2... gas sensors, 20... control circuit, 21, 26... AD converters, 22, 23... DA converters, 24... MPU, 25... power supply, MH1, MH2... heater resistors, R1... fixed resistor, Rd1 to Rd3... thermistors.

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 for controlling the first heater and the second heater, In a first period, the control circuit heats the second heater to a temperature higher than that of the first heater, and based on a detection signal appearing at the connection point between the first thermistor and the second thermistor in this state, generates an output signal representing the concentration of the gas to be measured, and makes the heating time of the first heater shorter than that of the second heater.

2. The gas sensor according to claim 1, wherein, In the first period, the control circuit makes the heating start timing of the first heater later than that of the second heater.

3. The gas sensor according to claim 2, wherein, In the first period, the control circuit makes the heating end timing of the first heater coincide with the heating end timing of the second heater.

4. The gas sensor according to any one of claims 1 to 3, wherein, In the first period, the control circuit changes the heating times of the first heater and the second heater based on the ambient temperature or the ambient humidity.

5. The gas sensor according to claim 1, wherein, In a second period, the control circuit heats the first heater to a temperature higher than that of the second heater, and makes the heating time of the second heater shorter than that of the first heater.

6. The gas sensor according to claim 5, wherein, In the second period, the control circuit makes the heating start timing of the second heater later than that of the first heater.

7. The gas sensor according to claim 6, wherein, In the second period, the control circuit makes the heating end timing of the first heater coincide with the heating end timing of the second heater.

8. The gas sensor according to any one of claims 5 to 7, wherein, In the second period, the control circuit changes the heating times of the first heater and the second heater based on the ambient temperature or the ambient humidity.

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