Gas detection system and gas detection method using metal oxide semiconductor gas sensors

Abstract

The introduction of an external atmosphere to the metal oxide semiconductor gas sensor is switched on / off by a valve. When the valve is closed and the gas sensor is isolated from the external atmosphere, the output of the gas sensor is stored as the reference output Rstd. The gas is detected from the reference output Rstd and the output Rs of the gas sensor at each point in time. [Effect] It is possible to sample the output of a gas sensor that corresponds to clean air with almost perfect accuracy.

Description

Technical Field

[0001] This invention relates to the detection of gases using a metal oxide semiconductor gas sensor.

Background Art

[0002] In the detection of gases using a metal oxide semiconductor gas sensor, it is known to sample the output corresponding to clean air from the output of the gas sensor as a reference output, and to detect the gas based on the change in the sensor output from the reference output. Such a gas detection method is called the "reference value update method". The reference output is the output corresponding to clean air in the environment where the gas sensor is placed, but it is unclear to what extent the sampled output is the output in a clean atmosphere.

[0003] Although not related to the reference value update method, Patent Document 1 (JP6268,484B) proposes providing a cap for a gas detector using a metal oxide semiconductor gas sensor and closing the cap when not in use. An adsorbent such as activated carbon is placed inside the gas detector, and the atmosphere is purified by the adsorbent when not in use to prevent poisoning of the gas sensor.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The problem of this invention is to sample the output of the gas sensor corresponding to clean air almost accurately when detecting a gas by the reference value update method.

Means for Solving the Problems

[0006] The gas detection system of this invention is A gas sensor comprising a metal oxide semiconductor whose resistance changes upon contact with a gas, and a heater for heating the metal oxide semiconductor, • A valve to turn the introduction of external atmosphere to the gas sensor on / off, The system includes a computer that processes the output of the gas sensor. The computer stores the output of the gas sensor as a reference output Rstd when the valve is closed and the gas sensor is isolated from the external atmosphere, and detects the gas from the reference output Rstd and the output Rs of the gas sensor at each point in time. The computer may be located in the same device as the gas sensor, or it may be located separately from the gas sensor and process the signals transmitted from the gas sensor.

[0007] The gas detection method of this invention uses a gas sensor having a metal oxide semiconductor whose resistance changes upon contact with a gas, and a heater for heating the metal oxide semiconductor. The introduction of an external atmosphere to the gas sensor is switched on / off by a valve, and a computer stores the output of the gas sensor when the valve is closed and the gas sensor is isolated from the external atmosphere as a reference output Rstd. Then, the gas is detected from the reference output Rstd and the output Rs of the gas sensor at each point in time.

[0008] The type of valve is arbitrary, but for example, a valve equipped with a shape memory alloy that opens and closes the valve seat depending on whether or not it is heated, and a heater that heats the shape memory alloy, can be used. The opening and closing of the valve seat controls the connection of the gas sensor to the external atmosphere. Instead of a valve using a shape memory alloy, a MEMS speaker with an array of piezoelectric elements may be operated at frequencies outside the range of human hearing. When the MEMS speaker is operated, the external atmosphere is introduced to the gas sensor, and when it is stopped, the gas sensor is isolated from the external atmosphere.

[0009] When the valve is closed and the gas sensor is isolated from the external atmosphere, the oxidative activity of the gas sensor causes flammable gases such as methane, hydrogen, and VOCs in the atmosphere to burn or decompose, and the atmosphere inside the gas sensor approaches a clean atmosphere. Therefore, if the output of the gas sensor with the valve closed, preferably the stable value of the gas sensor output with the valve closed, is used as the reference output, a gas sensor signal can be obtained in a clean atmosphere with almost no flammable gases. By combining the reference output with the output of the gas sensor at each point in time, gas can be detected more accurately.

[0010] The computer stores the moving average Ravg of the gas sensor output and detects the gas from the moving average Ravg, the reference output Rstd, and the gas sensor output Rs at each point in time. The ratio Rs / Ravg, which is the ratio of the gas sensor output Rs at each point in time to the moving average Ravg, represents the gas concentration assuming no gas is present in the recent average atmosphere. Rs / Rstd represents the gas concentration in the current atmosphere relative to a clean atmosphere, and Ravg / Rstd represents the gas concentration in the recent atmosphere relative to a clean atmosphere. Therefore, the gas concentration can be accurately determined from these signals. [Brief explanation of the drawing]

[0011] [Figure 1] Block diagram of the gas detection device in the example. [Figure 2] Diagram showing the gas sensor and valve in the embodiment. [Figure 3] A flowchart illustrating the gas detection method in the example. [Figure 4] This diagram shows the output waveform of the gas sensor when the valve is closed. [Figure 5] This diagram shows the output waveform of the gas sensor when the valve is opened. [Modes for carrying out the invention]

[0012] The following shows an optimal embodiment for carrying out the present invention. [Examples]

[0013] Examples are shown in Figures 1 to 5. Figure 1 shows a gas sensor detection device 2, in which the metal oxide semiconductor gas sensor 4 comprises a metal oxide semiconductor 5 whose resistance changes upon contact with gas, and a heater 6 that heats the metal oxide semiconductor 5. 8 is a switch for controlling the heater 6, and 10 is a load resistor connected to the metal oxide semiconductor 5. 12 is a microcomputer, 14 is a valve that turns the contact between the gas sensor 4 and the atmosphere on / off, and 16 is a display unit for the detection results.

[0014] The type of metal oxide semiconductor gas sensor 4 is arbitrary; for example, it can be a MEMS type, one using a flat substrate, or a bead-shaped sensor without a substrate. Also, since the gas sensor 4 is dependent on ambient temperature, the ambient temperature may be measured using a thermistor or similar device.

[0015] The microcomputer 12 includes, for example, a heater control 20 for controlling the heater 6, a processor 21, a program memory 22 for storing the operating program of the microcomputer 12, and an A / D converter 23 for performing A / D conversion on the output of the gas sensor 4 (voltage across the resistor 10). The data memory 24 stores the resistance value Rs of the gas sensor 4 (current resistance value of the metal oxide semiconductor 6), the moving average Ravg of Rs (the averaging interval is, for example, 10 minutes to 1 day), the resistance value Rstd (reference output) when the valve 14 is closed and the metal oxide semiconductor 5 is placed in a clean atmosphere, the time constant t1 of the resistance value of the metal oxide semiconductor 5 when the valve 14 is closed, the time constant t2 when the valve 14 is open, etc. The moving average Ravg can be a simple time average, or a weighted average in which, for example, higher resistance values ​​are given greater weight. The frequency of measuring the resistance value Rstd (reference output) of the metal oxide semiconductor 5 with the valve 14 closed is, for example, at least once a day and no more than once every three months, preferably at least once a week and no more than once a month.

[0016] The valve control unit 26 controls the valve 14, and the timer 28 counts the period the valve is closed, the elapsed time since the reference output Rstd was measured, etc. The I / O 30 performs input / output with an external device and outputs the detection results to the display unit 16.

[0017] For example, the ventilation fan 40 provided independently of the detection device 2 controls ventilation based on signals from the detection device 2 or the like. Also, a plurality of detection devices 2 are provided inside a building, factory, etc., and the detection results are notified to the server 42.

[0018] The gas detection device 2 performs, for example, measurement of VOCs, control of ventilation, detection of combustible gases, detection of toxic gases, etc., and the use of the gas detection device 2 is arbitrary. Instead of processing the signals of the gas sensor 2 by the microcomputer 12, the output of the gas sensor 2 or the like may be transmitted from the gas detection device 2 to the server 42, and the server 42 may perform sampling of the reference output, measurement of the moving average of the gas sensor output, detection of gases, etc.

[0019] Fig. 2 shows the gas sensor 4 and the valve 14. The gas sensor 14 includes, for example, a housing 50, a base 52, a stem 53, a substrate 54, and lead wires 55, and the metal oxide semiconductor 5 and the heater 6 are provided on the substrate 54.

[0020] The valve 14 uses, for example, the fact that a coil spring-shaped shape memory alloy 62 is deformed depending on the presence or absence of heating by the heater 60. In the valve 14, a valve body 64 is provided at the tip of the shape memory alloy 62. When the heater 60 is turned on, the shape memory alloy 62 extends and closes the valve seat 68, and when the heater 60 is turned off, the shape memory alloy 62 contracts and opens the valve seat 68. The valve 14 is provided with a flow path 66 to the outside and a flow path 69 to the gas sensor 4. When the valve seat 68 is opened, the flow paths 66 and 69 are connected, and when the valve seat 68 is closed, the flow paths 66 and 69 are separated. 70 is a substrate such as a printed circuit board, and 72 is a common housing for the gas sensor 4 and the valve 14.

[0021] The type of the valve 14 is arbitrary. For example, there is a MEMS element that vibrates a piezo thin film and generates a sound with an audible frequency. When the frequency of the generated sound is outside the audible range, it becomes an air pump. This MEMS element can also be used as a valve that introduces the external atmosphere during gas detection and blocks the gas sensor 4 from the outside during measurement of the reference output.

[0022] [[ID=第十九]] Figure 3 shows the gas detection and sampling algorithm for the reference output Rstd. Figure 4 shows the waveform of the resistance value of gas sensor 4 when the valve is closed, and Figure 5 shows the waveform of the resistance value of gas sensor 4 when the valve is open. The gas sensor used in Figures 4 and 5 is the applicant's TGS2600 gas sensor for detecting air pollution.

[0023] In gas detection, the output Rs (resistance value of the metal oxide semiconductor) of gas sensor 2 is measured, the moving average Ravg (time average of approximately 10 minutes to 1 day) of Rs is measured, and the reference output Rstd is read from the data memory. Rs / Ravg shows the change in the gas sensor output relative to the average atmosphere over the time range in which the moving average was calculated at that point. Furthermore, the output Rs of gas sensor 2 and the moving average Ravg are affected by factors other than the gas being detected (e.g., humidity), and if Rs is between Rstd and Ravg, it is difficult to detect the gas from Rs alone. Rs / Rstd shows the change in the gas sensor output relative to a clean atmosphere. Furthermore, Ravg / Rstd shows how far the average atmosphere deviates from clean air. Therefore, by comparing Rs with Ravg and Rstd, accurate gas detection is possible.

[0024] In measuring the reference output Rstd, valve 14 is closed to isolate the gas sensor 2 from the external atmosphere. The external atmosphere contains flammable gases such as methane, hydrogen, hydrocarbons, and VOCs at low concentrations. The gas sensor 2 keeps heater 6 on even after valve 14 is closed. Since the heated metal oxide semiconductor 4 has the ability to burn flammable gases, the concentration of flammable gases inside the gas sensor 2 decreases, and in the case of n-type metal oxide semiconductors 5 such as SnO2, In2O3, and WO3, the resistance value Rs increases. In the case of p-type semiconductors such as NiO and LaNiO3, the resistance value Rs decreases.

[0025] In Figure 4, after closing the valve, the resistance value Rs reaches a stable value in approximately 1000 seconds, and the stable value can be predicted after about 600 seconds have elapsed since closing the valve. Furthermore, by increasing the power of heater 6 after closing the valve to accelerate the combustion of the combustible gas, or by installing an oxidation catalyst and its heater inside gas sensor 4 and heating the oxidation catalyst when measuring Rstd, the stable value can be predicted in a short time, for example, 60 seconds after closing the valve. In this way, the time constant t1 of Rs and the stable value Rstd of Rs are measured when the valve is closed.

[0026] The time constant t2 of Rs when the valve is opened is measured. As shown in Figure 5, when the valve is opened, Rs decreases rapidly, and the time constant t2 can be easily measured. The time constant t1 represents the oxidative activity of the gas sensor 2, and t2 represents the response performance of the gas sensor 2. These time constants are then output to a server, for example.

[0027] For gas detection, for example, a two-dimensional table is prepared. One dimension corresponds to Rs / Rstd and the other dimension corresponds to Rs / Ravg. The values ​​in the table show the gas concentration for each pair of Rs / Rstd and Rs / Ravg. The values ​​read from the table are output to a server and displayed on a display. A one-dimensional table corresponding to Rs / Rstd would also be acceptable. [Explanation of symbols]

[0028] 2. Gas detection device 4. Metal oxide semiconductor gas sensor 5. Metal oxide semiconductors 6 Heaters 8 switches 10 resistors 12 Microcomputers 14 valves 16 Display 20 Heater control 21 processors 22 Program Memory 23 A / D converters 24 data memory 26 valve control 28 timers 30 I / O 40 Ventilation fan 42 servers 50 Housing 52 base 53 Stem 54 Substraight 55 Lead wires 60 Heater 62 Shape memory alloy 64 valve body 66, 69 channels 68 valve seats 70 circuit boards 72 Housing

Claims

1. In a gas detection system comprising a gas sensor having a metal oxide semiconductor whose resistance changes upon contact with a gas, and a heater for heating the metal oxide semiconductor, A valve that turns on / off the introduction of external atmosphere to the gas sensor, It includes a computer that processes the output of a gas sensor, The aforementioned computer, When the valve is closed and the gas sensor is isolated from the external atmosphere, the output of the gas sensor is stored as the reference output Rstd. A gas detection system that detects gas from a reference output Rstd and the output Rs of a gas sensor at each point in time.

2. The gas detection system according to claim 1, wherein the valve comprises a shape memory alloy that opens and closes a valve seat depending on whether or not it is heated, and a heater that heats the shape memory alloy, and the opening and closing of the valve seat controls the connection of the gas sensor to the external atmosphere.

3. The gas detection system according to claim 1 or 2, wherein the computer stores a moving average Ravg of the output of the gas sensor, and detects a gas from the moving average Ravg, a reference output Rstd, and the output Rs of the gas sensor at each point in time.

4. In a gas detection method that uses a gas sensor having a metal oxide semiconductor whose resistance changes upon contact with a gas, and a heater for heating the metal oxide semiconductor, The introduction of external atmosphere to the gas sensor is switched on / off by a valve. By computer, When the valve is closed and the gas sensor is isolated from the external atmosphere, the output of the gas sensor is stored as the reference output Rstd. A gas detection method that detects gas from a reference output Rstd and the output Rs of a gas sensor at each time point.

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