Gas sensor

Abstract

In a gas sensor having two thermistors connected in series, power consumption can be reduced. A gas sensor (1) is provided with thermistors (Rd1, Rd2), heater resistors (MH1, MH2) that heat the thermistors (Rd1, Rd2), respectively, and a control circuit (20). The control circuit (20), after starting heating the heater resistors (MH1, MH2), starts applying power to the thermistors (Rd1, Rd2), and in this state, generates an output signal (OUT) indicating the concentration of the gas to be measured on the basis of a detection signal (Vco2) occurring at a connection point between the thermistor (Rd1) and the thermistor (Rd2). Thus, since the timing of applying power to the thermistors (Rd1, Rd2) is delayed, the power consumed by the thermistors (Rd1, Rd2) can be 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 between a power supply and ground. 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] In the gas sensor described in Patent Document 1, power consumption is generated due to the current flowing through the two series-connected thermistors.

[0008] In the present invention, a technique for reducing power consumption in a gas sensor having two series-connected thermistors 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 that controls the first heater and the second heater and applies a power supply to the first thermistor and the second thermistor. During a first period, after starting the heating of the first heater and the second heater, the control circuit starts applying a power supply to the first thermistor and the second thermistor, and in this state, an output signal representing the concentration of the gas to be measured is generated based on a detection signal that appears at the connection point between the first thermistor and the second thermistor.

[0011] Effects of the Invention

[0012] According to the present invention, a technique for reducing power consumption in a gas sensor having two series-connected thermistors can be provided. Brief Description of the Drawings

[0013] Figure 1 It is a circuit diagram showing the structure of the gas sensor 1 according to the first embodiment of the technique of the present invention.

[0014] Figure 2 It is a timing chart for explaining the first operation example 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 the period T1.

[0018] Figure 6 It is a timing chart for explaining the second operation example of the gas sensor 1.

[0019] Figure 7 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.

[0020] Figure 8 It is a circuit diagram of the resistance measurement circuit 11.

[0021] Figure 9 It is a timing chart for explaining the operation of the gas sensor 2. Detailed Embodiment

[0022] Hereinafter, embodiments of the technology according to the present invention will be described in detail with reference to the accompanying drawings.

[0023] 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.

[0024] 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.

[0025] The thermistors Rd1 and Rd2 are detection elements made of materials such as composite metal oxides, amorphous silicon, polycrystalline silicon, and germanium that have a negative temperature coefficient of resistance. Both the thermistors Rd1 and Rd2 detect the concentration of CO2 gas, but as 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 the power supply 25 that supplies the power supply potential VDDS and the ground, and the detection signal Vco2 that appears at the connection point between the two is supplied to the control circuit 20.

[0026] 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 Vco2 that appears at the connection point between the thermistor Rd1 and the thermistor 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 and 23 apply a specified voltage to the heater resistors MH1 and MH2 by performing DA conversion on the digital value supplied from the MPU 24. That is, the heating temperature of the heater resistors MH1 and MH2 is controlled by the MPU 24.

[0027] Next, the operation of the gas sensor 1 of the present embodiment will be described.

[0028] Figure 2 It is a timing chart for explaining the first operation example of the gas sensor 1 of the present embodiment.

[0029] In Figure 2 In the first operation example shown, the gas measurement operation is performed during the period T1. In the gas measurement operation, the heater resistor MH1 is heated to 150 °C and the heater resistor MH2 is heated to 300 °C under the control of the MPU 24. 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.

[0030] 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 3In 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.

[0031] Figure 4 It is a graph showing the relationship between the temperatures of the thermistors Rd1 and Rd2 and the sensitivity to CO2 gas. As Figure 4 shown, the sensitivities of the thermistors Rd1 and Rd2 to CO2 gas vary greatly depending on the temperature, and in the temperature range below 40°C or above 300°C, 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 about 150°C.

[0032] Therefore, when there is CO2 gas in the measurement atmosphere while the thermistor Rd1 as 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 as 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 may also be almost non-existent.

[0033] Accordingly, the level of the detection signal Vco2 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 Vco2 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.

[0034] Here, in the gas measurement operation performed during period T1, the heating start timing of heater resistors MH1 and MH2 is time t11, the start timing of applying power from power supply 25 to thermistors Rd1 and Rd2 is time t12, and the heating end timing of heater resistors MH1 and MH2 and the end timing of applying power from power supply 25 to thermistors Rd1 and Rd2 are time t13. Accordingly, the heating time of heater resistors MH1 and MH2 is the period from time t11 to time t13, and the time of applying power to thermistors Rd1 and Rd2 is the period from time t12 to time t13. The time of applying power to thermistors Rd1 and Rd2 is sufficiently shorter than the heating time of heater resistors MH1 and MH2, for example, about 1 / 10. Thus, in the present embodiment, the timing of applying power to thermistors Rd1 and Rd2 is made later than the heating start timing of heater resistors MH1 and MH2.

[0035] Figure 5 It is a graph for explaining the time change of the heating temperature of thermistors Rd1 and Rd2 during period T1.

[0036] As Figure 5 shown, when the heating of heater resistors MH1 and MH2 starts at time t11, the temperatures of thermistors Rd1 and Rd2 rise. However, it takes a prescribed time for the temperatures of thermistors Rd1 and Rd2 to reach their respective target values of 150°C and 300°C. In Figure 5 the example shown, at time t1, the temperature of thermistor Rd1 reaches 150°C as the target value, and at time t2, the temperature of thermistor Rd2 reaches 300°C as the target value. That is, even after the heating of heater resistors MH1 and MH2 starts, the state in which the concentration of CO2 gas can be correctly measured by thermistors Rd1 and Rd2 does not exist during the period until time t2. The concentration of CO2 gas can be correctly measured after time t2 has passed. Therefore, even if power is applied to thermistors Rd1 and Rd2 before time t2, the detection signal Vco2 obtained before time t2 cannot be sampled. Considering this, in the present embodiment, the time t12 when power application to thermistors Rd1 and Rd2 starts is delayed, thereby enabling reduction of unnecessary power consumption.

[0037] The timing t12 at which power is applied to the thermistors Rd1 and Rd2 only needs to be after the timing t11 and before the timing t13, and is not particularly limited. However, by setting it to the timing t2 or its vicinity at which the concentration of CO2 gas can be correctly measured, the time for applying power to the thermistors Rd1 and Rd2 can be minimized. Then, the detection signal Vco2 obtained between the timing t12 and the timing t13 is sampled by the MPU24, and an output signal OUT representing the concentration of CO2 gas can be calculated based on this.

[0038] Here, the start and end of applying power to the thermistors Rd1 and Rd2 can be performed by the MPU24 controlling the generation and stop of the power potential VDDS of the power supply 25, or a switch can be provided at the output terminal of the power supply 25, and the switch can be turned on and off (ON / OFF) by the MPU24. In addition, the power supply 25 may not be a constant voltage source but a constant current source. Also, the heating start timing of the heater resistor MH1 and the heating start timing of the heater resistor MH2 do not need to be the same, and the heater resistor MH2 heated to a higher temperature can be made to start heating earlier than the heater resistor MH1.

[0039] In addition, the timing setting of the timing t12 is not particularly limited, and it can be the timing after a predetermined period has elapsed since the timing t11 when the heating of the heater resistors MH1 and MH2 starts. Thus, since the relationship between the timing t11 and the timing t12 is fixed, the control of the MPU24 becomes easy. Alternatively, the period from the timing t11 when the heating of the heater resistors MH1 and MH2 starts to the timing t12 when power is applied to the thermistors Rd1 and Rd2 can be made variable by the control of the MPU24.

[0040] In this way, in the first operation example, since power is applied to the thermistors Rd1 and Rd2 after the heating of the heater resistors MH1 and MH2 starts, the power consumption generated by the thermistors Rd1 and Rd2 can be reduced. Moreover, the self-heating amount of the thermistors Rd1 and Rd2 is also reduced, so the secular change of the thermistors Rd1 and Rd2 is also suppressed. In addition, it is not necessary to make the heating end timing of the heater resistors MH1 and MH2 exactly the same as the power application end timing of the thermistors Rd1 and Rd2, but by making the timings of the two the same, the generation of useless power consumption can be prevented.

[0041] Figure 6 It is a timing chart for explaining the second operation example of the gas sensor 1 of the present embodiment.

[0042] In Figure 6In the second operation example shown, a gas measurement operation is performed during period T1, and a virtual heating operation is performed during period T2. The gas measurement operation and the virtual heating operation are performed alternately. Since the gas measurement operation during period T1 is the same as that in the first operation example, repeated description is omitted.

[0043] As Figure 6 shown, in the virtual heating operation performed during period T2, the heater resistance MH1 is heated to 300°C and the heater resistance MH1 is heated to 150°C under the control of the MPU24. Thereby, the thermal history difference between the thermistors Rd1 and Rd2 caused by the gas measurement operation performed during period T1 is canceled. In order to cancel the thermal history difference more accurately, it is only necessary to make the length of period T1 the same as the length of period T2. In addition, since the detection signal Vco2 is not sampled during period T2, the power supply to the thermistors Rd1 and Rd2 can also be completely stopped. Thereby, power consumption can be further reduced. Or, as Figure 6 shown, the heating of the heater resistances MH1 and MH2 can also be started at time t21, the power supply to the thermistors Rd1 and Rd2 can be started at time t22, the heating of the heater resistances MH1 and MH2 can be ended at time t23, and the power supply 25 to the thermistors Rd1 and Rd2 can be ended. Thereby, the thermal history generated by the self-heating of the thermistors Rd1 and Rd2 during period T1 is the same as the thermal history generated by the self-heating of the thermistors Rd1 and Rd2 during period T2, and thus the thermal history difference between the two can be further reduced. In this case, it is preferable to make the power consumption and the power supply time of the thermistors Rd1 and Rd2 during period T1 the same as the power consumption and the power supply time of the thermistors Rd1 and Rd2 during period T2.

[0044] Figure 7 is a circuit diagram showing the structure of the gas sensor 2 according to the second embodiment of the technology of the present invention.

[0045] As Figure 7 shown, the difference between the gas sensor 2 of the second embodiment and the gas sensor 1 of the first embodiment is that resistance measurement circuits 11 and 12 and switches SW1 to SW3 are added. The other basic structures are the same as those of the gas sensor 1 of the first embodiment, so the same reference numerals are given to the same elements and repeated description is omitted.

[0046] 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 Vco2 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.

[0047] As Figure 8 shown in (a) of, the resistance measurement circuit 11 may also 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 directly measured resistance value of the thermistor Rd1.

[0048] Or, as Figure 8 shown in (b) of, it may also 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 directly measured resistance value of the thermistor Rd1.

[0049] The structure of the resistance measurement circuit 12 is the same, and the control circuit 20 can obtain the directly measured resistance value of the thermistor Rd2.

[0050] Figure 9 is a timing chart for explaining the operation of the gas sensor 2 of the present embodiment.

[0051] As Figure 9As shown, in this embodiment, switches SW1 to SW3 are opened at time point t11, whereby the MPU 24 monitors the resistance values R1 and R2 of the thermistors Rd1 and Rd2 measured by the resistance measurement circuits 11 and 12. Then, in response to the resistance value R1 of the thermistor Rd1 reaching a specified value R1th and the resistance value R2 of the thermistor Rd2 reaching a specified value R2th, switches SW1 to SW3 are closed, and power supply to the thermistors Rd1 and Rd2 is started. The specified value R1th is a threshold value that is exceeded when the thermistor Rd1 is heated to approximately 150°C regardless of the concentration of CO2 gas contained in the atmosphere. Similarly, the specified value R2th is a threshold value that is exceeded when the thermistor Rd2 is heated to approximately 300°C regardless of the concentration of CO2 gas contained in the atmosphere. Thereby, power supply to the thermistors Rd1 and Rd2 can be started in a state where the concentration of CO2 gas can be correctly measured by the thermistors Rd1 and Rd2.

[0052] However, it is not necessary to monitor both the resistance values R1 and R2 of the thermistors Rd1 and Rd2. It is also possible to monitor the resistance value of either one, and in response to it reaching a specified value, close switches SW1 to SW3 and start power supply to the thermistors Rd1 and Rd2. In this case, during the gas measurement operation period T1, it is also possible to monitor the resistance value of the thermistor Rd2 heated to a higher temperature, and during the virtual heating operation period T2, it is also possible to monitor the resistance value of the thermistor Rd1 heated to a higher temperature.

[0053] 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.

[0054] 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, when the gas to be measured is CO gas, a catalytic combustion type sensor unit can be used.

[0055] The technology according to the present invention includes the following structural examples, but is not limited thereto.

[0056] 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 that heat the first thermistor and the second thermistor respectively; and a control circuit that controls the first heater and the second heater and applies power to the first thermistor and the second thermistor. During a first period, after starting the heating of the first heater and the second heater, the control circuit starts to apply power to the first thermistor and the second thermistor, and in this state, 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. Thereby, the power consumption by the first thermistor and the second thermistor can be reduced.

[0057] In the above gas sensor, it may also be that during the first period, after a predetermined period has elapsed since the start of the heating of the first heater and the second heater, the control circuit starts to apply power to the first thermistor and the second thermistor. Thereby, the control of the control circuit becomes easier.

[0058] In the above gas sensor, it may also be that the control circuit controls the period from the start of the heating of the first heater and the second heater to the start of applying power to the first thermistor and the second thermistor during the first period. Thereby, power can be applied to the first thermistor and the second thermistor at the optimal timing.

[0059] In the above gas sensor, it may also be that during the first period, after starting the heating of the first heater and the second heater, the control circuit starts to apply power to the first thermistor and the second thermistor in response to the resistance value of the first thermistor or the second thermistor reaching a specified value. Thereby, power can be applied to the first thermistor and the second thermistor after obtaining a state where the concentration of the gas to be measured can be correctly measured.

[0060] In the above gas sensor, it may also be that the control circuit makes the heating end timing of the first heater and the second heater coincide with the power application end timing to the first thermistor and the second thermistor during the first period. Thereby, the control becomes easier and the generation of unnecessary power consumption can be prevented.

[0061] In the above gas sensor, it may also be that the control circuit heats the second heater to a higher temperature than the first heater during the first period and heats the first heater to a higher temperature than the second heater during the second period. Thereby, the difference in the thermal history between the first thermistor and the second thermistor can be reduced.

[0062] In the above-described gas sensor, alternatively, during the second period, after starting the heating of the first heater and the second heater, the control circuit starts applying power to the first thermistor and the second thermistor. Thereby, the difference in the thermal history due to self-heating of the first thermistor and the second thermistor is also reduced.

[0063] In the above-described gas sensor, alternatively, the control circuit makes the heating temperature and heating time of the first heater in the first period consistent with the heating temperature and heating time of the second heater in the second period, the control circuit makes the heating temperature and heating time of the second heater in the first period consistent with the heating temperature and heating time of the first heater in the second period, and the control circuit makes the power consumption and power application time of the first thermistor and the second thermistor in the first period consistent with the power consumption and power application time of the first thermistor and the second thermistor in the second period. Thereby, the difference in the thermal history between the first thermistor and the second thermistor can be reduced more accurately.

[0064] Description of symbols:

[0065] 1, 2... Gas sensors, 11, 12... Resistance measurement circuits, 11a... One end, 11b... The other end, 13... Constant current source, 14... Voltmeter, 15... Constant voltage source, 16... Ammeter, 20... Control circuit, 21... AD converter, 22, 23... DA converters, 24... MPU, 25... Power supply, MH1, MH2... Heater resistances, Rd1, Rd2... Thermistors, SW1 to SW3... Switches.

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 that controls the first heater and the second heater and applies power to the first thermistor and the second thermistor, In a first period, after starting the heating of the first heater and the second heater, the control circuit starts to apply power to the first thermistor and the second thermistor, and based on the detection signal appearing at the connection point between the first thermistor and the second thermistor in this state, an output signal representing the concentration of the gas to be measured is generated.

2. The gas sensor according to claim 1, wherein, In the first period, after a predetermined period has elapsed since the start of the heating of the first heater and the second heater, the control circuit starts to apply power to the first thermistor and the second thermistor.

3. The gas sensor according to claim 1, wherein, The control circuit controls the period from the start of the heating of the first heater and the second heater to the start of applying power to the first thermistor and the second thermistor in the first period.

4. The gas sensor according to claim 3, wherein, In the first period, after starting the heating of the first heater and the second heater, the control circuit starts to apply power to the first thermistor and the second thermistor in response to the resistance value of the first thermistor or the second thermistor reaching a specified value.

5. The gas sensor according to claim 1, wherein, The control circuit makes the heating end timing of the first heater and the second heater coincide with the power application end timing of the first thermistor and the second thermistor in the first period.

6. The gas sensor according to any one of claims 1 to 5, wherein, The control circuit heats the second heater to a higher temperature than the first heater in the first period, and heats the first heater to a higher temperature than the second heater in a second period.

7. The gas sensor according to claim 6, wherein, In the second period, after starting the heating of the first heater and the second heater, the control circuit starts to apply power to the first thermistor and the second thermistor.

8. The gas sensor according to claim 7, wherein, The control circuit makes the heating temperature and heating time of the first heater in the first period consistent with the heating temperature and heating time of the second heater in the second period. The control circuit makes the heating temperature and heating time of the second heater in the first period consistent with the heating temperature and heating time of the first heater in the second period. The control circuit makes the power consumption and power application time of the first thermistor and the second thermistor in the first period consistent with the power consumption and power application time of the first thermistor and the second thermistor in the second period.

Images