JP2003279518A - Carbon monoxide gas sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a CO sensor which saves electricity and which enhances a quantitative determination property with reference to CO. <P>SOLUTION: The sensor detects carbon monoxide gas on the basis of a resistance value of a filmlike oxide in a detection voltage application mode. The sensor is constituted in such a way that a purge application mode is composed of a first purge mode and a second purge mode, that the first-purge-mode ON time is in a range of 50 to 200 msec, that the second-purge-mode ON time is set to be larger than 100 msec and longer than the first-purge-mode ON time, that an electrical resistance value used to change over the first purge mode to the second purge mode is recognized in advance as a set value, and that the second purge mode is set when the electrical resistance value measured in the detection voltage application mode exceeds the set value. The gas sensor is constituted in such a way that the electrical resistance value is measured again in the second purge mode so as to decide whether the electrical resistance value exceeds the set value. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carbon monoxide gas (hereinafter also referred to as "CO gas" or simply "CO") sensor, and more specifically, it is excellent in power saving and quantification of gas (CO gas). Regarding sensors.

[0002]

2. Description of the Related Art In order to further popularize alarm devices for detecting household gas leaks and incomplete combustion, improvement in installation is strongly desired. In particular, making the alarm device cordless can be expected to greatly improve the installability, and therefore it is strongly desired to realize a battery-powered alarm device. The power consumption of a conventional gas sensor using a tin oxide sintered body and driven by a commercial power source is 200 mW to 1 W, but in order to realize a battery-powered alarm device with a life of 5 years, a drastic sensor from the current sensor is used. Improvements in structure and driving method are required. That is, in order to reduce the power consumption, it is important to improve the sensor structure (miniaturize, suppress heat dissipation) to miniaturize the heating portion and reduce the heat capacity.

The inventors of the present invention have disclosed in Japanese Patent Laid-Open No. 2000-29239.
Japanese Patent Publication No. 4 discloses a structure of a thin film gas sensor for achieving low power consumption in order to realize a battery-driven sensor. Here, a film-like oxide whose resistance value changes depending on the presence or absence of gas on a diaphragm-like support substrate in which the outer periphery or both ends of a thin film-like support film is supported by an electrically insulating substrate, and the above-mentioned film At least one pair of electrodes for measuring the electric resistance value of the oxide oxide and a heater for heating the film oxide are provided.

Generally, in a semiconductor type sensor for detecting CO, the sensor surface detects CO by the resistance value of the semiconductor when it is held at around 80 to 100 ° C., but when it is held at such a low temperature. However, since CO sensitivity disappears due to the adsorption of water, periodical purge heating for cleaning the surface is additionally required. To realize a battery-powered CO alarm and further extend the battery life,
It is important to reduce the electric power related to this periodic heating.

On the other hand, as a performance required for the CO alarm device, there is a quantitative property with respect to the CO concentration. That is, the degree of influence of CO on the human body depends on the product of its concentration and exposure time. Therefore, if the concentration is high, it is necessary to issue an alarm early, and if it is low, an alarm is issued after a certain period of time. If the concentration is below a certain level, it is not necessary to issue an alarm.

For example, according to the standards of the Japan Gas Equipment Inspection Association, when CO is 550 ppm, an alarm is issued within 5 minutes, when CO is 200 ppm, an alarm is issued within 15 minutes, and CO is 25 pp.
It is stipulated that there is no alarm below m.

We have optimized the tin oxide thin film, which is a gas-sensitive material, and the selective combustion catalyst layer in the structure disclosed in Japanese Unexamined Patent Publication No. 2000-292394, and have a quantitative property with respect to CO, that is, a change in sensor resistance value. The driving mode for maintaining a constant concentration gradient over a wide range with respect to the CO concentration,
We have conducted intensive research on this sensor structure.

FIG. 8 shows a flow of a method for driving a sensor using this structure. As a result, the purge temperature is set to 100 mse
When the value is set to be c or more, the resistance value of the sensor in the CO detection mode changes with a large concentration gradient in the CO concentration range of 30 to 500 ppm, but the purge temperature is 100 ms.
On the contrary, when it was set to be ec or less, it was found that the concentration gradient of the resistance change in this concentration range was small.

[Object of the Invention] The present invention has been made in view of the above situation, and an object thereof is to suppress power loss due to heat purge as much as possible in a diaphragm-like (like) CO gas sensor which is extremely power-saving. The present invention is to provide a CO sensor for simultaneously achieving the power saving and the improvement in the quantitativeness of CO by.

The present inventors set the purge temperature time to 1
When it was set to 00 msec or less, although it was unsuitable for quantitative detection, it was found that the operation mode was significant only in determining the presence or absence of CO. Therefore, the inventors have completed a CO alarm device for performing the following drive.

[0011]

According to a first aspect of the present invention, there is provided a CO gas sensor, wherein a thin film-shaped support film is provided with a gas on a support substrate in which an outer circumferential portion or both left and right ends are supported by electrically insulating substrates. A film-like oxide whose resistance value changes depending on the presence or absence of the film-like oxide, at least one pair of electrodes for measuring the electric resistance value of the film-like oxide, and a heater for heating the film-like oxide, The voltage application is performed in a predetermined cycle with a detection voltage application mode for detecting carbon monoxide gas, a purge voltage application mode for cleaning the sensor element surface, and a non-application mode in which no voltage is applied. In the sensor for detecting carbon monoxide gas based on the resistance value of the film oxide in the detection voltage application mode, the purge application mode is the first purge. It consists of over soil and the second purge mode,
In addition, the voltage application for alternately switching between these two modes is performed, the first purge mode ON time is set shorter than a predetermined time set in advance, and the second purge mode ON time is the first purge ON time. It is set longer than the time, and the electric resistance value for switching from the first purge mode to the second purge mode is recognized as a preset value in advance, and the detection voltage application mode and the purge voltage application mode are set. And when the electric resistance value of the film-shaped oxide measured in at least one mode selected from the group consisting of non-application modes exceeds the set value, the second purge mode is set, and the second purge mode is set. (2) In the purge mode, the electric resistance value is measured again, and it is determined whether or not the set value is exceeded, and if it is, an alarm is issued. .

A CO gas sensor according to a second aspect of the present invention is the sensor according to the first aspect, wherein the ON time of the first purge mode is set in the range of 50 to 200 msec, and the ON time of the second purge mode is 100 msec. It is set to be larger and longer than the ON time of the first purge mode.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As the purge mode, it is set to select either a first purge mode shorter than a predetermined time or a second purge mode longer than the first purge time. Detects with a sensitive sensor state even for low concentration CO by the first purge mode cycle.

The switching from the first purge mode cycle to the second purge mode cycle is at least one mode selected from a CO detection voltage application mode, a purge voltage application mode, and a mode in which no voltage is applied. It is determined by whether the electric resistance value of the film-shaped oxide in (3) exceeds a predetermined value. At this time, the CO concentration is determined by the resistance value in the CO detection mode when the second purge mode cycle is repeated. As a result, in an actual use environment, the time during which CO is generated is a small part of the total use time, so it is possible to reduce power consumption, and when CO is generated, the purge mode is switched to improve quantitativeness. It becomes possible to issue an alarm.

Here, the ON time of the first purge mode is 5
0 to 200 msec, the second purge mode ON time is longer than 100 msec, and
The time is set longer than the ON time of the purge mode. If the ON time of the first purge mode is shorter than 50 msec, the CO sensitivity is lowered with time, and therefore it is necessary to set it longer than this. However, 200 msec
Longer than 50 mse, the effect of reducing the power consumption by adopting the first purge mode is lost.
It is desirable to set in the range of c to 200 msec.

Further, the ON time of the second purge mode is 10
When it is shorter than 0 msec, the dependence of the sensor resistance on the CO concentration becomes small, and the CO = 100 ppm level and C
It cannot be distinguished from the O = 300 ppm level. In addition, when the second purge mode exceeds 1000 msec, the effect of improving the CO concentration dependency is reduced, which is not preferable.

Here, the voltage corresponding to the purge may be a voltage that leads to the surface temperature for cleaning the surface, and can be set to, for example, 400 ° C. or higher. In the sensor of this structure, after applying the purge voltage,
Since the temperature of the sensor surface reaches the surface temperature for purging in 2 msec, the ON time of the purging mode almost corresponds to the holding time of the purging temperature on the sensor surface.

As described above, the switching from the first purge mode cycle to the second purge mode cycle is performed in the CO detection voltage application mode, the purge voltage application mode, and the voltage non-application mode. It is determined by whether or not the change in the electric resistance value of the film oxide in at least one mode selected from the above exceeds a predetermined value.

In the case of an n-type semiconductor such as a tin oxide semiconductor, which has a film-like oxide whose resistance value decreases in the presence of CO gas, whether or not this electric resistance value becomes smaller than a predetermined value. Judge by Further, in the case of a p-type semiconductor such as copper oxide or nickel oxide, which is a film oxide whose resistance value increases in the presence of CO gas, it is determined by whether or not the electric resistance value becomes larger than a predetermined value. To do.

Next, the determination cycle of the sensor of the present invention will be described in detail below. In FIG. 1, switching from the first purge mode cycle to the second purge mode cycle is determined by the electric resistance value of the film oxide in the voltage application mode for CO detection. That is, an electric resistance value for switching from the first purge mode to the second purge mode is preset and recognized, and this set value is used as the detection voltage application mode and the purge voltage application mode. And when the electric resistance value of the film-shaped oxide measured in at least one mode selected from the group consisting of non-application modes exceeds, it proceeds to the second purge mode.

Here, the first purge mode time is 50 ms.
ec, the CO detection mode time was 500 msec, and the voltage non-application time was 20 seconds. In the detection cycle in the first purge mode, the transition to the second purge mode cycle is determined by the change in the resistance value in the CO detection mode.
The transition to the second purge mode cycle may be performed immediately after the end of the CO detection mode. However, according to the regular cycle up to that point, after the voltage non-application time has passed, the transition to the second purge mode cycle is performed. Good.

When shifting to the second purge mode cycle,
The relationship between the resistance value of the film oxide and the CO concentration in the CO detection mode is as shown in FIG. For comparison, FIG. 6 shows the relationship between the resistance value of the film oxide and the CO concentration in the CO detection mode of the first purge mode cycle. It can be seen that the shift to the second purge increases the concentration gradient from 30 ppm to 300 ppm and improves the quantitativeness.

The present inventors further conducted a study to save power by promptly switching from the first purge mode cycle to the second purge mode cycle. That is, it was found that the presence / absence of CO can be determined not only in the CO detection mode but also in the change of the resistance value during the first purge mode cycle or the change of the resistance value during the voltage non-application time.

FIG. 5 shows the atmospheric CO concentration dependency of the resistance value of the film oxide during the first purge mode cycle, during the purge mode and the mode in which no voltage is applied. As is clear from this figure, in any mode, a difference in resistance value is obtained between the air level and the level of CO = 30 ppm or more, and it is possible to determine the presence or absence of CO in these modes. It is possible.

FIG. 2 shows a determination cycle in the case of performing the determination based on the resistance value change in the first purge mode in addition to the determination in the voltage detection mode for CO detection.

With such a flow, the above-described determination of the extension of the purge time is made to the second purge mode cycle for more prompt quantitative determination of CO, as compared with the case where it is performed only in the CO detection mode. it can.

Further, FIG. 3 shows a cycle in which the shift to the second purge mode cycle is performed in addition to the change in the resistance value during the CO detection mode and the purge mode, and the judgment is made by the change in the resistance value during the voltage non-application time. ing. This further speeds up the transition to the second purge mode cycle,
It is possible to issue an alarm based on the concentration of CO at an early stage.

[0028]

EXAMPLES The details of the present invention will be specifically described below. <Method for Manufacturing Microsensor> FIG. 4 shows the structure of the thin film gas sensor used in the examples of the present invention. On the surface of a Si substrate having a thermal oxide film formed on both sides of 3000 angstroms, SiN and SiO ₂ to be a supporting layer of a diaphragm structure are formed.
The films were sequentially formed by the plasma CVD method to 1500 angstrom and 1 μm, respectively. A PtW film having a thickness of 0.5 μm was formed thereon as a heater layer, and a heater pattern was formed by wet etching. Further, after forming a SiO ₂ insulating film to a thickness of 2.0 μm by a sputtering method, the junction between the heater and the electrode pad was etched with HF to open a window.

Next, a Pt / Ta (2000 angstrom / 500 angstrom) film was formed as an electrode of the gas detection film and patterned by wet etching. Here, Ta serves as a bonding layer between the SiO ₂ and Pt films. Further, SnO ₂ by a sputtering method is used as a gas detection film on the top of this by 0.1 to 0.1% by a lift-off method.
It was formed with a thickness of 10 μm. Next, Pd on alumina particles
A powder supporting 0.1 to 7.5 wt% of the catalyst was made into a paste together with a binder, and SnO _{2 was formed} by screen printing.
Was applied to the surface of and was fired to form a selective combustion layer having a thickness of about 30 μm. Finally, Si is completely removed from the back surface of the substrate by dry etching to a size of 400 μm,
The diaphragm structure is used.

<Sensor Driving Method> An alarm operation circuit which operates according to the flowchart shown in FIG. 3 was produced. The heater ON time in the first purge mode is 50 msec, the CO detection mode time is 500 msec, and the OFF time is 149.
It is set to 45 seconds. Therefore, the cycle in the first purge mode of this embodiment is 2 minutes and 30 seconds.

It is desirable that the time of the first purge mode is shorter in order to save power, but if it is 50 msec or less, the effect of purging becomes almost zero. The upper limit may be selected from the viewpoint of power saving, and when it exceeds 200 msec, it is not necessary to set the second purge mode, so it is set within the range of 200 msec.

The determination to the second purge mode cycle is made by the CO
The determination is made in each of the detection mode and the voltage non-application mode, and this determination is made at the final point of each mode. Further, since the resistance value and the sensitivity of the film oxide in each voltage application mode are different, the determination level was determined from the air level and the CO sensitivity in each voltage application mode.

Second purge mode time is 200 msec
And The longer this time is, the more advantageous it is to increase the concentration gradient in the relationship between the output and the CO concentration, but
This effect becomes saturated when the time exceeds 400 msec.
Exceeding msec is not preferable from the viewpoint of power saving.

The determination of the sensor output in the CO detection mode in the second purge mode cycle may be set so that a different alarm is issued according to the output. When the output of the second purge mode becomes lower than the predetermined value, the first purge mode cycle is restored. Here, in the present embodiment, the return to the first purge mode cycle is made to shift to the heater OFF mode of the first purge mode, but other modes of the first purge mode (first purge mode and CO detection Mode).

In the CO detection mode in the second purge mode cycle, when a measured value of a predetermined value (for example, a CO concentration of 100 ppm or more is equivalent to sensitivity or more, specifically 10 kΩ or less in the following examples) is obtained, that is, CO When the existence is certain, the heater voltage OFF time may be set shorter than the heater voltage OFF time after the end of the CO detection mode in the first purge mode cycle. As a result, a more detailed alarm can be issued according to changes in the CO concentration of the environment.

In this embodiment, the detection cycle is set to 150 seconds even when the first purge mode cycle is changed to the second purge mode cycle.

Regarding the battery capacity, two alkaline AA batteries are used.
Each is equipped with an alarm action that can be confirmed by blinking an LED. CO concentration is 70 to 200 ppm, CO = 200
At about 500 ppm, different alarms depending on the concentration are obtained, such as one blinking every 5 seconds for the first alarm operation and three blinking every 5 seconds for the second alarm operation.

The alarm test was conducted as follows. An alarm was installed in the test chamber and a gassing test was performed in the next cycle. That is, CO = 30 ppm equivalent (10 minutes) → air (1 hour 20 minutes) → CO = 100p
pm equivalent (10 minutes) → air (1 hour 20 minutes) → CO =
A cycle of 300 ppm (10 minutes) → air (1 hour 20 minutes) was performed 100 times.

[Table 1] below shows the number of alarms issued and the battery level during the test period. An alarm device adopting the detection mode of the present invention and a detection mode shown in FIG. 8 for comparison (purge mode time is 50 msec, 100 msec, 200 ms).
The result of the alarm device which adopted 3 types of ec) is shown.

[0040]

[Table 1] It was found that the battery consumption after 100 times was 25.1 mWh in the conventional fixed pattern of 200 msec shown in Comparative Example 3. In the mode of the present invention, 12.6
It was mWh, and it was found that the consumption was about the same as when fixed at 50 msec (Comparative Example 1). Also,
As for the response to CO, the first alarm generated about 15 times the alarm for CO of 30 ppm, which should not be alarmed in Comparative Example 1, and when CO = 100 ppm, CO = 3.
It can be seen that the number of occurrences at 00 ppm is almost the same and it is difficult to distinguish between these concentrations.

In Comparative Example 2, no alarm can be maintained at the CO = 30 ppm level, and the CO = 100 ppm level,
It can be seen that the CO = 300 ppm level can be recognized with a probability of about 70%. Therefore, in the present invention, the second
The time of the purge mode was set to 200 msec, but when CO = 30 ppm, there is no alarm, and when 100 ppm and 300 ppm are recognized with a probability of more than half, 100 msec is set to the ON time of the second purge mode.
Should be set.

On the other hand, in the driving method of the present invention, CO = 30
In ppm, no alarm is given and CO =
Almost the first alarm at 100 ppm, CO = 300
It was found that the second alarm can be almost generated at the time of ppm, and the concentration between them can be distinguished. It was found that this characteristic is equivalent to the alarm operation function of Comparative Example 2 in which the power consumption is almost doubled.

[0043]

According to the present invention, it is possible to provide a CO sensor which can achieve both power saving and improvement in quantitativeness for CO.

[Brief description of drawings]

FIG. 1 is a chart showing a drive pattern by a sensor of the present invention.

FIG. 2 is a chart showing another drive pattern.

FIG. 3 is a chart diagram showing still another drive pattern.

FIG. 4 is an enlarged cross-sectional view showing a sensor element structure of the gas sensor of the present invention.

FIG. 5 is a diagram showing a relationship between a film oxide resistance value and a CO concentration in a first purge mode cycle of the sensor of the present invention (during a purge mode and a mode in which no voltage is applied).

FIG. 6 is a diagram showing a relationship between a film oxide resistance value and a CO concentration in the first purge mode cycle of the sensor of the present invention (during a CO detection mode).

FIG. 7 is a diagram showing a relationship between a film oxide resistance value and CO concentration in a second purge mode cycle of the sensor of the present invention (during CO detection mode).

FIG. 8 is a chart showing a conventional drive pattern.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Soichi Tabata             4-1-2 Hirano-cho, Chuo-ku, Osaka-shi, Osaka Prefecture               Within Osaka Gas Co., Ltd. (72) Inventor Hirokazu Sasaki             4-1-2 Hirano-cho, Chuo-ku, Osaka-shi, Osaka Prefecture               Within Osaka Gas Co., Ltd. (72) Inventor Hisao Onishi             4-1-2 Hirano-cho, Chuo-ku, Osaka-shi, Osaka Prefecture               Within Osaka Gas Co., Ltd. (72) Inventor Shinji Ogino             1-1 Tanabe Nitta, Kawasaki-ku, Kawasaki-shi, Kanagawa             Within Fuji Electric Co., Ltd. (72) Inventor Takuya Suzuki             1-1 Tanabe Nitta, Kawasaki-ku, Kawasaki-shi, Kanagawa             Within Fuji Electric Co., Ltd. (72) Inventor Ken Matsubara             1-1 Tanabe Nitta, Kawasaki-ku, Kawasaki-shi, Kanagawa             Within Fuji Electric Co., Ltd. (72) Inventor Kenji Kunihara             1-1 Tanabe Nitta, Kawasaki-ku, Kawasaki-shi, Kanagawa             Within Fuji Electric Co., Ltd. (72) Inventor Mitsuo Kobayashi             1-1 Tanabe Nitta, Kawasaki-ku, Kawasaki-shi, Kanagawa             Within Fuji Electric Co., Ltd. (72) Inventor Tokumi Nagase             1-1 Tanabe Nitta, Kawasaki-ku, Kawasaki-shi, Kanagawa             Within Fuji Electric Co., Ltd. F-term (reference) 2G046 AA11 BA01 BA04 BA09 BB02                       BB04 BB08 BE03 DB02 DB04                       DB05 DC07 DC14 FB02 FE38                       FE39

Claims (2)

[Claims] 1. A film-shaped oxide whose resistance value changes depending on the presence or absence of gas, on a support substrate in which the outer circumferential portion or both left and right ends of a thin film-shaped support film are supported by an electrically insulating substrate. At least one pair of electrodes for measuring the electrical resistance value of the film oxide and a heater for heating the film oxide are provided, and voltage application to the heater is for detecting carbon monoxide gas. The detection voltage application mode, the purge voltage application mode for cleaning the surface of the sensor element, and the non-application mode in which no voltage is applied are performed by repeating at a predetermined cycle. In the sensor for detecting carbon monoxide gas based on the resistance value of the film-shaped oxide in the above, the purging application mode includes a first purging mode and a second purging mode, and The voltage application for alternately switching the modes is performed, the first purge mode ON time is set shorter than a preset predetermined time, and the second purge mode ON time is longer than the first purge ON time. The electrical resistance value for switching from the first purge mode to the second purge mode that has been set is recognized as a preset value in advance, and the detection voltage application mode, the purge voltage application mode, and the non-application When the electric resistance value of the film-shaped oxide measured in at least one mode selected from the group consisting of modes exceeds the set value, the second purge mode is set, and the second purge mode is set. Then, the electric resistance value is measured again to judge whether or not the set value is exceeded, and if it exceeds the set value, an alarm is issued. Carbon gas sensor. 2. The ON time of the first purge mode is 50 to 50.
The carbon monoxide sensor according to claim 1, wherein the carbon monoxide sensor is set to a range of 200 msec, the ON time of the second purge mode is set to be longer than 100 msec and longer than the ON time of the first purge mode.