JP2805048B2 - Gas detector - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a gas detection device using an output waveform of a gas sensor accompanying a temperature change.

[Prior Art] JP-A-58-189,547 discloses that a gas sensor is changed in temperature and gas is detected from a waveform of a sensor output accompanying the temperature change. In this publication, an output waveform of a sensor at the time of a temperature change is stored, and a gas is detected by comparing the output waveform with a previously stored waveform. However, this publication does not show a specific method of analyzing the obtained waveform.

When the waveform of the sensor output with respect to the temperature change is clearly different for each type of gas, there is no problem even if the waveform analysis method is not particularly considered. However, if the difference between the waveforms for each gas is unclear, it is difficult to identify the type of gas. Storing the entire output waveform with respect to a temperature change increases the storage capacity of the device.

[Problems of the Invention] The objects of the present invention are: (1) To establish a method of analyzing the output waveform of a sensor accompanying a temperature change to facilitate detection of the type and concentration of gas, and in particular, the characteristics of the output waveform are ambiguous. In such a case, the type of gas can be determined. (2) The storage capacity of the detection device must be reduced,
In

[Configuration of the Invention] In the present invention, the output of the gas sensor at the time of temperature change is differentiated with respect to time. Then, the type and concentration of the gas are detected from the position of the peak of the differentiation and the value of the sensor output at at least one point due to the temperature change. The peak position of the differential output sharply reflects the gas type, and the gas type can be determined from the peak position. If the type of gas can be determined, the gas concentration is determined from, for example, a sensor at the end of the temperature change. In addition to the peak position of the differential value, its intensity, for example, the size and area of the peak can be used as a signal indicating the type of gas. Further, the position of the peak of the differential output differs depending on the concentration even for the same gas. Therefore, it is preferable to correct the model value at the peak position according to the gas concentration.

Example A SnO ₂ gas sensor (gas sensor “TGS812” of the applicant, TGS812 is a trade name) was used as a gas sensor, and the heater was exposed to a cycle in which the heater was turned on for 1 second at a cycle of 7 seconds and turned off for 6 seconds. As a result, the sensor changes temperature between room temperature and 500 ° C. 100 to 10,000p for each gas to be detected
PM, methane, isobutane, carbon monoxide, and hydrogen were selected, and these gas species were identified and their concentrations were determined.

FIGS. 3A to 3D show output waveforms of the temperature change sensors for these gases. The circuit conditions used for the measurement will be described later together with the description of FIG. Fig. 3 (A)-
The vertical axis of (D) represents the sensor output v, and the horizontal axis represents the time from heater ON. The waveform in the figure is the gas concentration from the bottom.
Corresponds to 100 ppm, 300 ppm, 1000 ppm, 3000 ppm, 10,000 ppm.

From the waveforms in the figure, it is difficult to determine the type of gas. The difference between the methane isobutane waveforms is ambiguous only in the magnitude of the output. The waveforms of hydrogen and carbon monoxide are similar. For example, it is difficult to distinguish between an output waveform for 1000 ppm of hydrogen and an output waveform for 10,000 ppm of carbon monoxide.

4 (A) to 4 (E) show differential waveforms of the sensor output. These differential waveforms correspond to 3,000 ppm of each gas,
The waveform for two seconds after turning on the heater is shown in the figure. The vertical axis in the figure represents the time differential value of the sensor output, which is shown by changing the magnification according to the figure. That is, air, methane, and carbon monoxide with small sensor outputs are enlarged and displayed. Note that the data in this figure does not directly correspond to FIG. 3 because the sensors used are different. As is clear from the figure, the differential output has a peak, and the position differs depending on the gas. For example, in the case of air or methane, a peak appears just before the end of the temperature rise. On the other hand, for isobutane, the middle of the heating cycle (about 1/2 second)
A peak occurs. In the case of hydrogen, a peak occurs in the first half of the heating cycle. For carbon monoxide, a peak occurs in the first half of the heating cycle, but the peak is broad, weak, and unclear.

5 (A) to 5 (D) show time derivative waveforms of the sensor output for each gas of 100 to 10,000 ppm. These data are obtained by converting the data shown in FIGS. 3A to 3D into differential values. FIGS. 5 (A) to 5 (D) also show different magnifications for each gas type and concentration. In the case of methane, the peak is located immediately before the end of the heating cycle, and in the case of isobutane, the peak is located almost in the middle of the heating cycle. The peaks of hydrogen and carbon monoxide are near the first half of the heating cycle. And the peak for carbon monoxide is weak. Further, even with the same gas, the position of the peak differs depending on the concentration. The dependence of the peak position on the gas concentration is shown by the chain line in the figure.

From these results, it is understood that the type and concentration of the gas can be determined by the following processing. The absolute value of the sensor output is sampled at at least one point, and the model value at the peak position of the differential output is calculated for each gas type so as to match the sampled absolute value. The calculated model value is compared with the actually measured peak position, and it is assumed that the best matching gas is generated. Thus, the type of gas is identified. Once the type of gas is determined, the concentration is determined from the absolute value of the output.

FIG. 1 shows a circuit configuration of the embodiment. In the figure, 2
Denotes a gas sensor, 4 denotes a heater thereof, and 6 denotes a gas sensing part using a metal oxide semiconductor such as SnO ₂ or In ₂ O ₃ . The type of the gas sensor 2 is arbitrary, and may be any as long as it has a heater and a gas sensitive part. Further, as the gas sensing unit, a unit that burns a gas with a catalytic combustion catalyst and detects the combustion heat, for example, may be used. Alternatively, a proton conductor such as antimonic acid may be used.

8 is an appropriate power supply, 10 is a switch such as an FET, 12 is a load resistor, and 14 is an NTC thermistor for compensating the ambient temperature dependency of the gas sensor 2. Here, a cycle of taking out an output of 10 V from the power supply 8 and applying the output to the heater 4 for 1 second and turning off the heater 4 for 6 seconds was used. Metal oxide semiconductor 6
And the load resistance 12 is 5V, and the load resistance 12 is 3K
Ω. C ₁ is a differentiation capacitor, 16 is a resistor for extracting a differential output, and C ₂ is a differential output smoothing capacitor.

20 is a microcomputer and 22 is an A / D converter. In the A / D converter 22, the input is switched by a built-in switch, and both the differential output of the voltage to the load resistor 12 and the differential output to the resistor 16 are A / D converted. Reference numeral 24 denotes an ALU (arithmetic logic operation unit), and reference numeral 26 denotes a ROM, which stores, in addition to the operation program, model values such as an absolute value of a sensor output and a peak position of a differential output for each gas type and concentration. When these model values are stored in the ROM 26 for an arbitrary gas concentration, the ROM
26 storage capacity increases. So, for example, 100ppm, 300pp
Model values were stored for five points of m, 1000 ppm, 3000 ppm, and 10,000 ppm.

A data storage RAM 28 stores the absolute value of the sensor output, the peak position τ of the differential output, the height P thereof, and the like. In the embodiment, the sensor output S at the end of the temperature change and the sensor output s before the start of the temperature change are stored as the absolute value of the sensor output. Among them, the gas concentration was determined from the sensor output S at the end of the temperature change, and the sensor output s before the start of the concentration change was used for checking the reliability of the measurement result. Similar results can be obtained by using the maximum value of the sensor output during the heating process or the sensor output at a specific time from the start of heating, instead of the output S. Although the peak position τ is stored as the time at which the peak occurs, it may be stored as a sensor output when the peak occurs. The ROM 26 stores model values such as S, s, P, and τ for specific concentrations. Thus, using the actually measured output S, model values such as s, P, and τ for an arbitrary S are created. This creation is performed by, for example, using the model values of the two points closest to S and interpolating the model values by linear approximation or the like. The interpolation is performed as follows, for example. Output S 2
The two model values are internally divided by the measured value of S. Next, the model values of τ, P, and s are internally divided according to the internal division ratio. The model values such as τ, P, etc. obtained for each gas so that S matches each other are also stored in the RAM 28. In addition, the RAM 28 also stores detection results such as the type and concentration of gas. 30 is a timer for controlling the heater 4.

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

The operation of the apparatus will be described with reference to FIG. The timer 30 is reset and restarted. Here, the time signal is T. Then, for the first second, the heater 4 is turned on using the switch 10.
Turn on. Further, the sensor output s before the start of the temperature rise is sampled before the heater 4 is turned on. This signal s is equal to the minimum value of the sensor output because the sensor output increases when the temperature rise is started. Next, the time differential value of the sensor output during the temperature rising process is sampled. The sampling is performed on the peak position τ and the peak height P of the differential output.

After a lapse of one second, the heater 4 is turned off, and thereafter, the heater 4 is turned off for six seconds. The sensor output S at the end of the temperature rise is sampled and stored. During the period when the heater is off, operations such as identification of a gas type and determination of a gas concentration are performed. First, a model value of the peak position τ, the height P, and the sensor output s before the start of temperature rise is created for each type of gas using the sensor output S at the end of temperature rise. Then, the type of gas is determined from the peak position τ. For example, in the case of methane, isobutane, hydrogen and carbon monoxide, three groups of methane, isobutane, hydrogen and carbon monoxide can be identified from the peak position τ. Note that the peak position τ varies depending on the concentration even with the same gas.
Are changed according to the gas concentration (actually, the sensor output S at the end of the temperature rise) and stored. Whether it is hydrogen or carbon monoxide can be identified by using the peak height P or the peak broadness. Thus, the type of gas is determined.

After the type of gas is determined, the model value of s is compared with the actually measured value to determine whether the error is within an allowable range. If the error is within the allowable range, the gas concentration is determined from S, and the type and concentration of the gas are displayed. If the error exceeds the allowable range, an error is displayed. Of course, such a check may be omitted.

After these, when the time of the timer 30 reaches 7 seconds, the process shifts to the next detection cycle.

The embodiment is merely an example, and various modifications are possible. For example, the cycle of the temperature change may be about 1 msec to 10 minutes. The reason for using the 7-second cycle of heater-on for 1 second and heater-off for 6 seconds in the embodiment is that there is a fear that the gas concentration may change in the course of temperature change. Therefore, a preferable time for the temperature change is 10 seconds or less, more preferably 5 seconds or less, which is used for creating a derivative (1 second in the embodiment).

In the embodiment, four kinds of gases of methane, isobutane, carbon monoxide, and hydrogen are targeted, but the target gas is arbitrary.
If a sensor having high water vapor sensitivity is used, it is possible to detect not only combustible gas and toxic gas but also water vapor. Further RO
If the types of model values stored in M26 are increased, analysis of mixed gas is also possible.

The sensor output to be used is not limited to the temperature rising side, but may be the temperature falling side, or both the temperature rising side and the temperature falling side. The condition and the range of the temperature change of the sensor are arbitrary, and for example, a temperature change between 300 ° C. and 500 ° C. or a temperature change between 300 ° C. and room temperature may be used.

In the embodiment, were subjected to analog differentiator using a capacitor C _1, it may be digital differential. In the case of digital differentiation, it goes without saying that various processing such as smoothing of the differential output may be performed.

[Effect of the Invention] In the present invention, the type of gas is identified from the peak position of the time derivative of the sensor output with respect to the temperature change. The peak position of the differential output differs depending on the type of gas, and it is easy to identify the type of gas. Also, the signal to be sampled is not the entire output waveform at the time of temperature change, but only a specific signal such as a peak, so that the storage capacity required for the device can be reduced.

When the type of gas is ambiguous only at the position of the peak of the differential output, if the peak intensity such as the height or area of the peak is used,
The type of gas can be identified. The position of the peak depends on the gas concentration. If this is taken into account and the model value for the peak position is determined, the reliability of detection can be further improved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of an embodiment, and FIG. 2 is an operation flowchart thereof. 3 (A) to 3 (D) are characteristic diagrams of a conventional example,
FIGS. 4 (A) to (E) and FIGS. 5 (A) to (D)
It is a characteristic view of an Example. In the figure, 2 ... gas sensor, 20 ... microcomputer, 22 ... A / D converter, 24 ... ALU, 26 ... ROM, 28 ... RAM, 30 ... timer, 34 ... display.

Claims (4)

(57) [Claims] 1. An apparatus for changing a heating temperature of a gas sensor having a heater and a gas sensing portion and detecting gas from an output of the gas sensor at the time of temperature change, wherein at least one point of the gas sensor at the time of temperature change Sampling means for storing the output, time-differentiating the gas sensor output at the time of temperature change, and a differential value storing means for storing the peak position; and storing the stored gas sensor output and the peak position of the time differential value as a gas type. A gas detection device for comparing a gas value and a model value stored in advance in correspondence with the gas concentration and an identification means for obtaining a gas type and a gas concentration. 2. The differential value storage output stage stores, in addition to the peak position of the time differential value of the gas sensor output, the peak intensity, and the identification means stores the sensor intensity stored in the sensor output storage means. The peak position and its intensity stored in the output / differential value storage means are compared with model values stored in advance corresponding to the type and concentration of gas. Gas detector. 3. The gas detection device according to claim 1, wherein the gas sensor uses a metal oxide semiconductor whose resistance value changes with gas as a gas sensing portion. 4. The gas detecting apparatus according to claim 2, wherein said identification means stores a model value of a peak position of a time differential value by changing the model value according to a gas concentration. .