JP7659469B2 - Gas sensor and method for correcting gas sensor - Google Patents

Description

The present invention relates to a gas sensor and a method for correcting a gas sensor.

Since the output (sensor sensitivity) of a gas sensor changes due to deterioration over time, the sensor sensitivity is corrected in response to the change due to deterioration over time (see, for example, Patent Documents 1 and 2). Patent Document 1 describes a method of using a gas sensor element equipped with a measurement detection section and a correction detection section, and correcting the sensor sensitivity of the measurement detection section in response to a comparison result of the outputs of both detection sections. Patent Document 2 describes a method of storing in advance in a ROM (Read Only Memory) the relationship between the air base output, the amount of time elapsed, and the sensitivity of the gas sensor in a catalytic combustion type gas sensor, and acquiring the amount of time elapsed and the sensor sensitivity from the ROM when the difference between the output for the gas to be detected during combustion and the air base output is equal to or less than a certain value to correct the sensor sensitivity.

Patent No. 3167798 JP 2008-267810 A

The correction method described in Patent Document 1 requires multiple detection units for measurement and correction, which increases costs. In addition, trace amounts of flammable gases (VOCs: Volatile Organic Compounds) and water vapor are present in the environment, and the environmental temperature also changes, and minute changes in these (concentration, humidity, temperature) cause fluctuations in the air base output. The correction method described in Patent Document 2 does not take into account fluctuations in the air base output due to environmental influences, and is therefore inferior in detection accuracy for the amount of change due to deterioration of sensor sensitivity over time.

In view of the above circumstances, the present invention aims to provide a gas sensor and a method for correcting a gas sensor that can appropriately correct the output of the gas sensor in response to changes in the output of the gas sensor due to deterioration over time while suppressing increases in costs.

The gas sensor of the present invention is a gas sensor equipped with a correction unit that corrects the sensor output or the threshold value for determining the detection of the test gas. The correction unit acquires an air base output, which is the sensor output in an environment in which the test gas is not present, determines the degree of the decrease in the fluctuation or fluctuation range of the air base output, and corrects the sensor output or the threshold value for determining the detection of the test gas according to the determined degree of the decrease in the fluctuation or fluctuation range of the air base output.

The gas sensor correction method of the present invention is a gas sensor correction method that uses a computer to correct the sensor output or the threshold value for determining the detection of the test gas, and obtains the air base output, which is the sensor output in an environment in which the test gas is not present, and calculates the degree of the decrease in the fluctuation or fluctuation range of the air base output, and corrects the sensor output or the threshold value for determining the detection of the test gas according to the degree of the decrease in the fluctuation or fluctuation range of the air base output.

The present invention makes it possible to appropriately correct the output of the gas sensor in response to changes in the output of the gas sensor due to deterioration over time, while suppressing increases in costs.

FIG. 1 is a diagram showing a schematic configuration of a gas sensor according to an embodiment of the present invention. FIG. 2 is a graph showing the air-based output of a gas sensor. FIG. 3 is a graph showing the relationship between the fluctuation of the air base output and the output voltage. FIG. 4 is a graph showing the relationship between the rate of change of the fluctuation of the air base output and the rate of deterioration of the output voltage.

The present invention will be described below in accordance with a preferred embodiment. Note that the present invention is not limited to the embodiment described below, and can be modified as appropriate without departing from the spirit of the present invention. In addition, in the embodiment described below, some configurations are omitted from illustration and description, but for the details of the omitted technology, publicly known or well-known technology is applied as appropriate within the scope of not causing any inconsistencies with the contents described below.

Figure 1 is a diagram showing the schematic configuration of a gas sensor 1 according to one embodiment of the present invention. The gas sensor 1 shown in this figure is a catalytic combustion type (or adsorption combustion type) gas sensor equipped with a bridge circuit 40 with a detection element 20 and a compensation element 30 as two sides. Examples of test gases detected by this gas sensor 1 include methane, hydrogen, propane, and butane. Note that the adsorption combustion type is a gas detection method in which molecules of the test gas are adsorbed to the catalyst of the detection element 20 while the pulse voltage is OFF or LOW voltage, and when the pulse voltage becomes ON or HIGH voltage, the gas molecules are burned on the catalyst of the detection element 20, resulting in a sensor output.

The detection element 20 includes a heater 21 and a detection section 22 that covers the heater 21. The compensation element 30 includes a heater 31 and a compensation section 32 that covers the heater 31. Examples of the heaters 21 and 31 include microheaters patterned on a microchip. Examples of the material of the heaters 21 and 31 include platinum (Pt).

The detection unit 22 is made of a material having catalytic activity in an oxidation reaction. The detection unit 22 includes a catalyst and a catalyst carrier that supports the catalyst. Examples of the catalyst material of the detection unit 22 include platinum group metals such as platinum, palladium (Pd), iridium (Ir), ruthenium (Ru), and rhodium (Rh), and other metals such as gold (Au), silver (Ag), copper (Cu), nickel (Ni), and cobalt (Co). Examples of the catalyst carrier material of the detection unit 22 include alumina (Al ₂ O ₃ ), titanium oxide (TiO ₂ ), cerium oxide (CeO ₂ ), and tin oxide (SnO ₂ ). The compensation unit 32 is made of a material that does not have catalytic activity in an oxidation reaction. The compensation unit 32 is made of the same material as the catalyst carrier of the detection unit 22.

In the gas sensor 1, when molecules of a combustible test gas come into contact with (or are adsorbed on) the detection section 22, which is heated to several hundred degrees by the heater 21, a catalytic combustion reaction (or an adsorption combustion reaction) occurs on the detection section 22, and the heat of the reaction increases the temperature of the heater 21, increasing its resistance value. On the other hand, even if molecules of a combustible test gas come into contact with (or are adsorbed on) the compensation section 32 of the compensation element 30, no catalytic combustion reaction (or an adsorption combustion reaction) occurs. In this type of gas sensor 1, the change in the resistance value of the heater 21 is obtained as a sensor output by the bridge circuit 40 to detect the test gas or identify the concentration of the test gas.

The bridge circuit 40 includes a pair of parallel circuits 40A, 40B and a power source 41. The pair of parallel circuits 40A, 40B branch off from a conductor connected to the positive electrode of the power source 41. The pair of branched parallel circuits 40A, 40B are reconnected to a conductor connected to the negative electrode of the power source 41 via a variable resistor VR. In one parallel circuit 40A, the detection element 20 and the compensation element 30 are connected in series. In the other parallel circuit 40B, a pair of fixed resistors R1, R2 are connected in series. In this embodiment, the detection element 20 is arranged on the positive electrode side of the power source 41, and the compensation element 30 is arranged on the negative electrode side of the power source 41.

The bridge circuit 40 includes a terminal 42 connected between the detection element 20 and the compensation element 30, and a terminal 43 connected between the fixed resistors R1 and R2, and outputs an output voltage _Vout , which is a potential difference between the terminals 42 and 43, to an MCU (Micro Controller Unit) 50. The MCU 50 pulse-drives the bridge circuit 40 and detects the test gas based on the output voltage _Vout of the bridge circuit 40. As will be described later, the MCU 50 also corrects the output voltage _Vout in response to fluctuations in the air base output _VAB .

2 is a graph showing the air base output V _AB [mV] of the gas sensor 1. The air base output V _AB shown in this graph is the output voltage V _out in an environment where no gas to be detected is present. Since trace amounts of flammable gas and water vapor are present in the environment and the environmental temperature also changes, minute changes in these (concentration, humidity, temperature) cause fluctuations in the air base output V _AB . Note that the "fluctuation" of the air base output V _AB does not refer to long-term fluctuations in the air base output V _AB relative to its initial value, but refers to short-term fluctuations in the air base output V _AB , for example, over 600 seconds.

The MCU 50 periodically acquires the air base output V _AB and determines the fluctuation of the air base output V _AB . The acquisition of the air base output V _AB continues for 600 seconds at 10 second intervals, for example. The fluctuation of the air base output V _AB can be exemplified by the standard deviation, root mean square fluctuation, or variance of the air base output V _AB over a short period of time, and can be determined from a large number of values acquired at 10 second intervals over a period of 600 seconds, for example.

The MCU 50 obtains an initial value of the fluctuation of the air base output _VAB and stores it in a memory (not shown). The MCU 50 periodically obtains the fluctuation of the air base output _VAB , compares it with the initial value stored in the memory, and obtains the degree of decrease of the fluctuation of the air base output _VAB from the initial value. Then, the MCU 50 corrects the output voltage _Vout according to the obtained degree of decrease of the fluctuation of the air base output _VAB from the initial value.

3 is a graph showing the relationship between the fluctuation (μV) of the air base output V _AB and the output voltage V _out (mV). This graph shows the relationship between the fluctuation (μV) of the air base output V _AB and the output voltage V _out (mV) when the gas sensor 1 is installed in an environment where the concentration of the detected gas is 1000 ppm. As shown in this graph, the fluctuation of the air base output V _AB and the output voltage V _out are proportional to each other, and the larger the fluctuation of the air base output V _AB , the higher the output voltage V _out .

Here, at the beginning of use of the gas sensor 1, the catalyst in the detection section 22 of the detection element 20 maintains high activity, so the air-based output V _AB is relatively high, and the fluctuation of the air-based output V _AB due to environmental influences is also relatively large. In contrast, as the gas sensor 1 is used for a long period of time, the air-based output V _AB (sensitivity) becomes relatively low due to deterioration of the catalyst in the detection section 22 of the detection element 20 (such as adsorption and adhesion of molecules with large heat capacity and poisonous substances to the catalyst), and the fluctuation of the air-based output V _AB due to environmental influences also becomes relatively small. In other words, there is a correlation between the degree of decrease in the fluctuation of the air-based output V _AB and the degree of decrease (deterioration rate) of the output voltage V _out (sensitivity).

4 is a graph showing the relationship between the degree of decrease (rate of change) in the fluctuation of the air base output V _AB and the degree of decrease (rate of deterioration) in the output voltage V _out (sensitivity). As shown in this graph, the rate of change in the fluctuation of the air base output V _AB and the rate of deterioration of the sensitivity are in a proportional relationship, and the greater the rate of change in the fluctuation of the air base output V _AB , the higher the rate of deterioration of the output voltage V _out .

Therefore, in the gas sensor 1 of this embodiment, the MCU 50 calculates the rate of change of the fluctuation of the air base output _VAB from its initial value, and corrects the output voltage _Vout of the gas sensor 1 by multiplying the output voltage _Vout by a correction coefficient according to the calculated rate of change.

As described above, in the gas sensor 1 of this embodiment, the MCU 50 acquires the air base output V _AB , calculates the degree of reduction in the fluctuation of the air base output V _AB , and corrects the output voltage V _out in accordance with the calculated degree of reduction in the fluctuation of the air base output V _AB . This makes it possible to correct the output voltage V _{out taking into account the influence of deterioration over time of the sensor output (sensitivity) for the detected gas without providing a sensor element for calibration (with only a sensor element for detection). Therefore, it is possible to appropriately correct the output voltage V out} of _the gas sensor 1 in accordance with the change due to deterioration over time of the output voltage V _out of the gas sensor 1, and to suppress an increase in the cost of the gas sensor 1.

In addition, the output voltage Vout of the gas sensor 1 can be appropriately corrected in accordance with the change due to deterioration over time of the output voltage _Vout of the gas sensor 1, without using any calibration gas and without an operator having to go to the installation site of the _gas sensor 1.

The present invention has been described above based on the above embodiment, but the present invention is not limited to the above embodiment, and modifications may be made without departing from the spirit of the present invention, and publicly known or well-known technologies may be appropriately combined.

For example, in the above embodiment, the output voltage _Vout is corrected according to the degree of reduction in the fluctuation of the air base output _VAB , but instead of correcting the output voltage _Vout , the threshold value for determining the presence or absence of the detected gas may be corrected. In this case, the greater the degree of reduction in the fluctuation of the air base output _VAB , the lower the threshold value for determining the presence or absence of the detected gas.

In the above embodiment, the degree of reduction in the fluctuation of the air base output V _AB is obtained, and the output voltage V _out is corrected in accordance with the obtained degree of reduction in the fluctuation of the air base output V _AB . However, instead of the fluctuation of the air base output V _AB , the degree of reduction in the fluctuation range, which is the difference between the maximum and minimum values of the air base output V _AB , may be obtained, and the output voltage V _out and the threshold value for determining the presence or absence of the test gas may be corrected in accordance with the obtained degree of narrowing of the fluctuation range of the air base _output V AB. When the fluctuation range of the air base output V _AB is obtained, the accuracy of the correction of the output voltage V _out and the threshold value can be improved by performing a process such as removing singular values.

In the above embodiment, the catalytic combustion type (or adsorption combustion type) gas sensor 1 is used as an example, but the present invention can also be applied to other types of gas sensors, such as semiconductor type and hot wire semiconductor type. Furthermore, the gas sensor of the present invention can be applied not only to gas alarms, but also to various gas detectors such as portable checkers and stationary devices.

1 Gas sensor 50 MCU (correction unit, computer)
V _AB Air base output V _out Output voltage (sensor output)

Claims (3)

A gas sensor including a correction unit that corrects a sensor output or a threshold value for determining detection of a test gas,
The correction unit is
An air base output, which is a sensor output in an environment in which the test gas is not present, is obtained, and a degree of reduction in the fluctuation or fluctuation range of the air base output is obtained;
The gas sensor corrects a threshold value for determining whether the sensor output or the detection of the test gas is detected, depending on the degree of the fluctuation or the decrease in the fluctuation range of the air base output. The gas sensor according to claim 1, wherein the correction unit stores an initial value of the fluctuation or fluctuation range of the air base output, and determines the degree of decrease in the fluctuation or fluctuation range of the air base output relative to the initial value. A gas sensor calibration method for calibrating a sensor output or a threshold value for determining detection of a test gas using a computer, comprising the steps of:
An air base output, which is a sensor output in an environment in which the test gas is not present, is obtained, and a degree of reduction in the fluctuation or fluctuation range of the air base output is obtained;
A gas sensor correction method for correcting a sensor output or a threshold value for determining detection of a test gas in accordance with the degree of fluctuation or decrease in the fluctuation range of the air base output.

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