JPH01311260A - Gas detecting device - Google Patents

Abstract

PURPOSE:To decide the kind and concentration of gas by comparing the features of the histogram of the output of a gas sensor at the time of temperature variation with previously stored features. CONSTITUTION:A gas sensing part 6 is a metal oxide semiconductor which varies in resistance value by sensing gas and a gas sensor 2 consists of the gas sensing part 6 and a heater 4. Then the heater 4 is turned on for a specific time and then turns off for a specific time to vary the temperature of the sensing part 6, and an output waveform accompanying the temperature variation is converted into a histogram to have a clear peak, thereby extracting features. Here, the ROM 26 of a microcomputer 20 is stored with features of a histogram by the kinds and concentration of the gas, the RAM 28 for data storage is stored with the histogram of the output of the sensor 24 and its features, and the extracted features are compared with the previously found features to find the kind and concentration of the gas.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a gas detection device that uses an output waveform of a gas sensor that accompanies temperature changes.

[Prior Art] JP-A-58-189.547 discloses changing the temperature of a gas sensor and detecting gas from the waveform of the sensor output accompanying the temperature change.

In this publication, the output waveform of the sensor at the time of temperature change is stored and compared with a pre-stored waveform to detect gas.

However, this publication does not disclose a specific analysis method for the obtained waveform. It is not shown how to identify the type of gas, especially when the waveform characteristics are ambiguous.

Furthermore, if the entire output waveform of the sensor is used, it becomes necessary to store a signal corresponding to the entire output waveform in advance on the detection device side as well. Is this generally a considerable storage capacity?
I need.

[Problem to be solved by the invention 1 The problems to be solved by this invention are as follows: (1) To establish a method for analyzing the output waveform of a sensor due to temperature changes, and to facilitate the detection of the type and concentration of gas;
In particular, it is possible to distinguish the type of gas even when the characteristics of the output waveform are ambiguous; and (2) it is possible to reduce the memory capacity required for the detection device.

[Structure of the Invention] In the present invention, the output of a gas sensor accompanying a temperature change is converted into a histogram, and features are extracted from the histogram.

If a histogram is used, features can be extracted more easily than if the output waveform is used as is. For example, Figure 3 (A)
The output waveforms of ~(E) are ambiguous. However, when these waveforms are converted into histograms as shown in FIGS. 5(A) to 5(E), clear peaks appear. Therefore, for example, this peak is used as a characteristic indicating the type of gas.

Next, the extracted features are compared with features previously determined for each gas type and concentration to determine the gas type and concentration. As the characteristics, for example, the maximum value of the sensor output when the temperature changes, the presence or absence of a peak in the histogram, its position, and the size of the peak are used. As the size of the peak, for example, peak height or peak area may be used. Further, the gas sensor may be one in which a gas sensitive part using a metal oxide semiconductor whose resistance value changes depending on gas, such as 5n02, is heated by a heater.

In this invention, features extracted from the histogram are compared with features previously stored in the device.

Therefore, the information for gas identification that is stored in advance in the device may be only the characteristics of the histogram, and there is no need to store the entire waveform of the sensor output. Furthermore, the histogram may be sampled with enough precision to extract features. For these reasons, the storage capacity required for the detection device can be reduced.

[Example] A SnO2-based gas sensor (Ikken's ff gas sensor TGS 812", T'GS 812 is a trade name) was used as a gas sensor, and the heater was turned on for 1 second in a 7-second cycle.
It was subjected to a cycle where the heater was turned off for 2 seconds. As a result,
The sensor changes temperature between room temperature and 500°C. Methane, isobutane, -carbon oxide, hydrogen, and ethanol were selected as gases to be detected, each having a concentration of 100 to 10.000 ppm, and the types of these gases were identified and their concentrations were determined. For ethanol only, the concentration to be measured is 100 to 3000p.
It was set as pm.

The output waveforms of the sensor when the temperature changes for these gases are shown in FIGS. 3(A) to 3(E). The circuit conditions used for the measurements will be shown later along with the explanation of FIG.

The vertical axes in FIGS. 3(A) to (E) represent the sensor output V, and the horizontal axes represent the time since the heater was turned on. In addition, the waveforms in the figure are gas concentrations of 1100pp, 300ppm, l from the bottom.
000ppm, 3000ppm.

10, corresponds to OOOppm.

It is difficult to determine the type of gas from the waveform in the figure. The waveforms of methane and isobutane differ only in the magnitude of the output, and the difference is ambiguous. Also, the waveforms of hydrogen and ethanol are similar. -The waveform for carbon oxide is intermediate between that for hydrogen and ethanol and that for isobutane.

Histograms of sensor output are shown in FIGS. 4 and 5 (A) to (E). These histograms are obtained by converting the output waveform of FIG. 3 into histograms, and are histograms of the sensor output for 1 second after the heater is turned on. Of course, a histogram on the temperature decreasing side may be used instead of the histogram on the temperature increasing side, or a histogram on both the temperature increasing side and the temperature decreasing side. The horizontal axis of the figure represents the sensor output, with the left end representing the output 0 and the right end representing the full scale (the full scale value differs from time to time and is expressed as f and s for each time). The vertical axis is the frequency of each output. FIG. 4 shows a histogram of clean air, and FIG. 5 shows a histogram of each gas.

From these figures it is clear that: For air and methane, there is only one peak, and the peak is located on the low output side. For other gases such as isobutane, -carbon oxide, hydrogen, and ethanol, peaks appear at two locations, one on the low output side and one on the high output side.

Furthermore, the relative magnitudes of the two peaks vary depending on the gas concentration, but also vary depending on the type of gas. For isobutane, the peak on the low output side is noticeable, and for -carbon oxide and ethanol, the peak on the high output side is noticeable. Hydrogen shows an intermediate pattern. Furthermore, the interval between peaks varies depending on the type of gas. For example, 10.000 ppm carbon monoxide and 300
When comparing ppm hydrogen and ethanol, hydrogen can be distinguished from the strong peak on the low output side. Next, for carbon monoxide, the interval between peaks is narrow, and for ethanol, the interval between peaks is large. In addition to this information, the maximum value and minimum value of the sensor output, or the interval between the maximum value and the minimum value, etc. can also be used. Therefore, by using these signals, the type and concentration of gas can be determined. For example, the maximum value of the sensor output may be used as information regarding the gas concentration, and the number of peaks, their positions, and their sizes may be used as information regarding the type of gas.

FIG. 1 shows the circuit configuration of the embodiment. In the figure, 2 is a gas sensor, 4 is its heater, and 6 is 5nOz, In2
This is a gas sensitive part using a metal oxide semiconductor such as O.

The type of gas sensor 2 is arbitrary as long as it has a heater and a gas sensing section. Furthermore, the gas sensing section may be configured to combust gas using a catalytic combustion catalyst and detect the heat of combustion. Alternatively, a proton conductor such as antimonic acid may be used.

Reference numeral 8 designates a suitable power source, IO a switch such as an FET, 12 a negative resistance, and 14 an NTC thermistor for compensating for the ambient temperature dependence of the gas sensor 2.

Here, a cycle was used in which the IOV output was taken out from the power source 8, applied to the heater 4 for 1 second, and the heater 4 was turned off for 6 seconds. Further, the voltage applied to the metal oxide semiconductor 6 and the load resistor 12 was 5V, and the load resistors 1 and 2 were set to 3Ω.

20 is a microcomputer, 22 is an A/D converter, 24 is an ALU (arithmetic logic unit), and 26 is a ROM in which operating programs and histogram characteristics for each gas type and concentration are stored. A data storage RAM 28 stores a histogram of the sensor output, features extracted from the histogram, theoretical values for each type of gas for these features, and detection results such as the type and concentration of gas.

As the characteristics of the histogram, for example, the height PL% of the peak on the low output side of the histogram, its position v1, the height P2 of the peak on the high output side, and its position v2 are used.

In addition to this, the maximum value S and minimum value S of the sensor output V are used. The signals S, s may be obtained from the histogram,
Alternatively, it may be determined directly from the waveform without using a histogram. If theoretical values for these characteristics are stored in the ROM 26 for each arbitrary gas concentration, the storage capacity of the ROM 26 will increase. So, for example, loopppm, 300ppm,
loooppm, 3000ppm, lo, 000ppm
Theoretical values are stored for the five points, and interpolation is performed between them by linear approximation or the like. For example, the interpolation is performed using the maximum value S of the sensor output.
is used to internally divide the maximum value S stored for each gas concentration by the actual measured value of S. Next, according to this internal division ratio, Pl, P2
, vl, ■3, S, etc. are internally divided among the stored data. 30 is a timer for controlling the heater 4.

32 is a clock circuit, and 34 is a display for displaying detection results.

The operation of the device will be explained with reference to FIG. Reset the timer 30 and restart it. Here, let T be the time signal. Then turn on heater 4 using switch lO for the first 1 second.
Turn on and create a histogram of the sensor output during that time. For example, the histogram is created by dividing the detection voltage of 5V by 100 and creating it every 50mV. Further, the sensor output V is sampled at 400 points at intervals of 2°5m5ec, for example.

The heater 4 is turned off after 1 second has elapsed, and then the heater 4 is turned off for 6 seconds. During the period when the heater is off, the type of gas can be identified,
Performs work such as determining gas concentration.

First, characteristics such as the number, position, and size of the maximum value S1 and minimum value s1 of the sensor output are extracted. Low output peak
P is determined from between the minimum value S and (s+S)/2, and the peak P2 on the high output side is determined from between (S+S)/2 and S. If there is no peak on the high output side (methane),
Let P2 be O.

Next, using S, P L, P 2, V l for each type of gas
, V2, create theoretical values for S. This operation uses the values stored in the ROM 26 for each gas type and concentration, and calculates the values for each concentration by internally dividing them so that S matches.
After these operations, determine the type of gas. peak P
, the gas is methane. Otherwise, P! The type of gas is determined from the ratio of /Pl and the value of lv+V21.

Specifically, for example, the errors between the actual measured values and the theoretical values are determined, the degree of the error is evaluated by the least squares method, and it is assumed that the gas with the smallest least squares error is generated.

Once the type of gas can be determined, the maximum value S and minimum value s of the output can be determined.
The gas concentration can be determined from the position, height, etc. of 1 or each peak. Here, the gas concentration is determined from the maximum value S.

In the example, the minimum value S of the sensor output is unused. So S, P,,P,,VlsV! , or use only S to confirm the rationality of the detection results. For example, the error between these signals and the theoretical value is evaluated using the least squares method, and if the error is within an allowable range, it is considered reasonable, and the type and concentration of the detected gas are displayed.

If the error exceeds the allowable range, an error message will be displayed.

Of course, the rationality test may be omitted.

After these steps, when the timer 30 reaches 7 seconds, the next detection cycle begins.

The embodiment is merely an example, and various modifications are possible.

For example, the temperature change cycle may be about 1 m5 ec to 10 min. The reason why a 7 second cycle of heater on for 1 second and heater off for 6 seconds was used in the example was because it was feared that the gas concentration would change during the process of temperature change. Therefore, the preferable temperature change time is 10 seconds or less, more preferably 5 seconds or less, which is the time used to create the histogram (1 second in the example).

Examples include methane, isobutane, carbon oxide, hydrogen,
Five types of gas, including ethanol, were targeted, but the target gas is arbitrary. Furthermore, if a sensor with high water vapor sensitivity is used, it is possible to detect not only flammable gases and toxic gases but also water vapor. Furthermore, by increasing the amount of information stored in the ROM 26, it is also possible to analyze mixed gases.

The histogram is not limited to the temperature increasing side, but may also be used on the temperature decreasing side or both the temperature increasing side and the temperature decreasing side. The conditions and range of the temperature change of the sensor are arbitrary, for example, a temperature change between 300°C and 500°C, or a temperature change between 300°C and room temperature,
Any of the following may be used.

Further, the histogram may be a histogram of differential values of the sensor output, and processing such as smoothing or data compression may be applied to create the histogram.

[Advantageous Effects of the Invention 1] In this invention, a method for analyzing sensor output with respect to temperature changes is established, and the type and concentration of gas can be easily determined. In particular, when a histogram is used, the characteristics of each gas become clearer than when the sensor output waveform is used as is. Further, as the theoretical value for each gas type to be stored in the device, only its characteristics may be used instead of the entire histogram. Therefore, the storage capacity required for the device can be reduced.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of the embodiment, and FIG. 2 is an operation flowchart thereof. Figures 3 (A) to (E) are characteristic diagrams of conventional examples;
FIG. 4 and FIGS. 5(A) to 5(E) are characteristic diagrams of the embodiment. In the figure: 2 gas sensor, 20 microcomputer, 22 A/D converter, 24 ALU, 26 ROM. 28 RAM, 30 timer, 34 display.

Claims (4)

[Claims] (1) In a device that detects gas from the output of the gas sensor when the temperature changes by changing the heating temperature of a gas sensor having a heater and a gas sensing part, means for determining a histogram of the gas sensor output when the temperature changes; The present invention is characterized by providing a means for extracting the characteristics of the obtained histogram, and a means for comparing the extracted characteristics with characteristics stored in advance corresponding to the type and concentration of gas to determine the type and concentration of the gas. , gas detection equipment. (2) The gas detection device according to claim 1, characterized in that the maximum value of the gas sensor output, the presence or absence of a peak in the histogram, its position, and its size are used as the characteristics of the histogram. (3) The gas detection device according to claim 1 or 2, characterized in that the gas sensor uses a gas sensing portion made of a metal oxide semiconductor whose resistance value changes depending on the gas. (4) The histogram is characterized in that the height or area of the peak is used as the size of the peak.
The gas detection device according to claim 2 or 3.