JPH01316651A - Method and device for detecting gas - Google Patents

Abstract

PURPOSE:To save the power consumption of a gas sensor by constituting the gas sensor by connecting a detection electrode to a semiconductor layer of a metal oxide and intermittently heating the gas sensor by means of heating pulses, with the sensor being left at about room temperature while it is not heated. CONSTITUTION:A metallic exothermic body 4 is coated with a heat-resisting insulating coating and a semiconductor layer 6 of a metal oxide 6 and at least one electrode are provided on the coating. Gas detection is made from the resistance value of the semiconductor layer 6. The metallic exothermic body 4 is intermittently caused to produce heat by means of heating pulses of <=2 seconds in width and <=1/5 in duty ratio through a transistor Tr by actuating an oscillation circuit Osc 2 with the output pulse of another oscillation circuit Osc 1. While no heating pulse is applied, this gas sensor 2 is left at about room temperature and gas detection is made from the resistance value of the semiconductor layer 6.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to gas detection using a change in the resistance value of a metal oxide semiconductor. This invention particularly relates to reducing the power consumption of the gas sensor used.

[Prior Art] In order to reduce the power consumption of gas sensors, the inventors
-Developed a gas sensor in which a heating element such as a Cr-Al alloy or a Ni-Cr alloy is coated with a heat-resistant insulating coating such as alumina and used as a carrier for a metal oxide semiconductor layer (for example, Patent Application No. 61-2
56,082-086, Japanese Patent Application No. 16.844.1988, Japanese Patent Application No. 174.219-220.). These gas sensors use a metal heating element with a wire diameter of about 20 μm, which is covered with a heat-resistant insulating coating of alumina or the like with a thickness of about 100 A to 5 μm. Then, a metal oxide semiconductor layer and an electrode are provided on the insulating coating so that the resistance value of the metal oxide semiconductor layer can be detected. Here, the insulating coating is used for insulation between the metal oxide semiconductor layer and the heating element. Of course, the metal heating element may also be a plate-shaped metal heating element (for example,
, No. 844). These gas sensors are intended for light chain thermal capacity and power consumption of the sensor.

Currently (E), even the best gas sensor consumes less power (e.g. 3
The power consumption when heating to 00℃) is about 40 mWatL, and the thermal time constant (after heating starts, the steady heating temperature is 9
The time required to raise the temperature to 0%) is about 100 m5ec.

The inventor studied the relationship between heating time and characteristics of a gas sensor and found that the sensor operates even if it is heated for only a very short period of time (see FIG. 4). For example, once every second 5
A heating pulse of approximately 11 sec-100 m5 ec is applied to heat the sensor. The resistance value at the end of the heating pulse is then measured. Even under such conditions, the resistance value of the sensor changes depending on the gas, and the gas can be detected. Further, the width of one heating pulse may be shorter than the thermal time constant of the sensor.

For example, a gas sensor with a thermal time constant of 100 m5ec is
The sensor operates even when heated with the pulse width of ISeC.

Therefore, by applying heating pulses to the sensor intermittently and making the pulse width sufficiently short, power consumption will be reduced accordingly. For example, if the heating pulse is set to 5111 Sec/sec, the power consumption will be reduced to about 1/100 compared to the case of continuous heating.

In addition, related prior art will be shown here. Among the inventors' earlier applications, Japanese Patent Application No. 61-256,084 points out that the metal heating element of the gas sensor has a low resistance, making it difficult to obtain an adequate power source. This application proposes that the metal heating element be energized in a pulsed manner. However, the pulse interval here is shorter than the thermal time constant of the sensor, and the senna is kept at a constant heating temperature.

Next, Japanese Patent Publication No. 53-43,320 discloses that the temperature of a gas sensor is alternately changed between a high temperature range and a low temperature range, and carbon monoxide is selectively detected from the output in the low temperature range. The heating time to the high temperature range here is sufficient time for heat cleaning of the sensor, and the current heating time is 10 seconds to 20 seconds.
It is understood that more than seconds.

[Invention Section Ml An object of the present invention is to further reduce the power consumption of a gas sensor.

[Structure and operation of the invention] In the present invention, a heat-resistant insulating coating is applied to the surface of a metal heating element to serve as a carrier for a metal oxide semiconductor layer.

A detection electrode is then connected to the metal oxide semiconductor layer to form a gas sensor. Next, this gas sensor is intermittently heated by heating pulses and left at around room temperature during the heating period. For example, during a period when no heating pulse is applied, the power applied to the metal heating element is set to 0, and heating is performed only by the heating pulse. Of course, during the period when no heating pulse is applied, it is sufficient to continue applying power to the metal heating element to an extent that does not contribute to the overall power consumption. The shorter the width of the heating pulse, the lower the power consumption of the sensor, so the pulse width is 2 seconds or less, preferably 17z seconds or more.
2 seconds, more preferably 50 microseconds to 200 mseconds. The duty ratio of the pulse is 115 or less, preferably 1/2
Must be 0 or less. Also, the interval between pulses is 001
The time is preferably from seconds to 10 minutes, more preferably 0.5
2 seconds to 2 minutes.

By performing such short-term intermittent heating, the power consumption of the gas sensor can be reduced.

Even if the gas sensor is heated intermittently for very short periods of time, the sensor will still operate. Therefore, gas is detected from the resistance value during heating (when the sensor temperature rises). Gas can also be detected from the resistance value when the sensor cools down after heating. Such detection is useful, for example, in the detection of carbon oxides. That is, heat cleaning of the sensor can be performed even by heating for a very short time.

[Example] FIG. 1 shows a circuit diagram of an example. In the figure, Eb is an appropriate power supply, and here it is assumed to be a 3V power supply of 1.5Vx2. This is intended for battery use of the device. Osc
1 is an oscillation circuit that generates a pulse with a width of 10 m5ec once every second, and 0sc2 is an oscillation circuit that generates a pulse with a width of 25 μSee every 500 μSee. and oscillation circuit 0sc
The oscillation circuit 0s uses the output pulse of l as a strobe signal.
oscillate c2. 0scl.

0sc2 constitutes a pulse power supply. However, when a logic circuit such as a microcomputer is used as the auxiliary circuit, the pulse power source may be built into the logic circuit such as the microcomputer.

2 is a gas sensor in which a heat-resistant insulating coating is applied to the surface of a metal heating element 4, and a metal oxide semiconductor layer 6 and at least one electrode are provided on the coating. In the gas sensor used, the metal heating element 4 was made of Fe-Cr-A with a wire diameter of 20 μm.
l alloy wire is used, and the metal oxide semiconductor layer 6 is made of plain SnO.
, and the heat-resistant insulating coating has a thickness of about 1 μm.
It was made into an O3 film. In this sensor 2, the power consumption during continuous heating to 300°C is 70 mWalt, and the applied voltage is approximately 0.6
(2) The resistance value of the heating element 4 was 5Ω. Further, the ripening time constant is 100II+sec. When using one electrode, the metal heating element 4 is also used as one electrode. Tr is a transistor connected to the metal heating element 4, and can be changed to any switch.

1'tl is the load resistance of the sensor 2, and the voltage across it is the sensor output Vout.

A1. A2 is an operational amplifier, D1 to D3 are diodes, C
I is a capacitor, R2-R4 are resistors, and these constitute a peak hold circuit. That is, the peak of the output Vout is accumulated in the capacitor C1, and is transferred to the operational amplifier A2.
Take it out. R5 is the output resistance of the operational amplifier A2, and M is a meter for displaying the detection results. Load resistance R1
Therefore, the meter M is used as a gas detection means. The reason for using the peak hold circuit here is as follows. The sensor output Vout varies depending on the heating pulse. The signal with the most alpha flavor is usually the signal at the point near the end of the heating pulse where the sensor temperature is the highest.

At this time, due to the temperature dependence of the sensor, the output Vou
t indicates a peak. This peak is then captured and processed by gas detection means. Furthermore, the duration of the peak of Vout is generally short, and signal processing is facilitated by holding the peak with a peak hold circuit.

FIG. 2 shows a circuit that specifies the sampling timing of the sensor output V out. In this circuit, the oscillation circuit Osc
At the end of the strobe signal of l, the A/D conversion circuit A/
Edge trigger D. Note that the A/D conversion circuit A/D preferably has a built-in sample and hold circuit.

Unless a sample and hold circuit is included, a high-speed A/D conversion circuit will be required because the sampling time of Vout will be short. Such circuits are generally expensive. And A
The A/D converted output is displayed on the display circuit D1play, and the output of the A/D conversion circuit is taken out as a control output Control. In this circuit, gas is detected based on the resistance value of the metal oxide semiconductor layer 6 at the end of the heating pulse. By changing the timing of the output sampling signal, the sensor output V out at any time can be taken out. For example, the output immediately before the heating pulse ends may be sampled, or the output may be sampled when the sensor 2 has cooled to room temperature.

FIG. 3 shows the operation of the circuit of FIG. 1. Oscillation circuit 0sc
The oscillation circuit 0sc2 is operated with an output pulse of 1, and the metal heating element 4 is heated via the transistor Tr. The duty ratio of the oscillation circuit Osc 2 is l/20, and the output of the power supply Eb is 3V, so this is the output pulse of the oscillation circuit 0scl (
Metal heating element 4 at a voltage of 0.7v between
is equivalent to heating. Doing it like this is 3v or 5
This is because it is easier to obtain a power source such as V than a 0°7V power source. The output pulse of the oscillation circuit 0scl is once per second and has a width of 1
0 m5ec, and the power consumption of the sensor 2 is reduced to about 1/100 compared to the case of continuous heating.

The heating pulse to sensor 2 has a thermal time constant (100 msec)
It is shorter than . For this reason, the heating temperature of sensor 2 does not reach the steady value (this does not reach, however, the power supply Eb and the oscillation circuit Oscl 5
If the pulse width of Osc2 is stable, the heating pattern is constant and the temperature of sensor 2 changes in the same pattern. That is, even if the heating pulse is made shorter than the thermal time constant, the heating pattern of the sensor 2 is stable. Next, the resistance value of the metal oxide semiconductor layer 6 is influenced by the gas even by heating for a time shorter than the thermal time constant or 2 seconds or less, so that the gas can be detected. Therefore, the output Vout of the sensor 2 is sampled at an appropriate time to detect the gas. In the case of Figure 1, sensor 2
Sample the peak of the output.

FIG. 4 shows the relationship between the width of the heating pulse of the sensor 2 and the resistance value. The horizontal axis represents the voltage applied to the metal heating element 4, and the vertical axis represents the resistance value of the metal oxide semiconductor layer 6 immediately before the end of the heating pulse. Note that the atmosphere is 20° C., humidity is 65%, and the temperature during continuous heating is approximately 300° C. at an applied voltage of 0.6 V. From the figure, (
1) gas can be detected even if the heating pulse width is shortened; (2) the pulse width can be shorter than the thermal time constant; It can be seen that higher applied voltages are preferred.

FIG. 5 shows the response characteristics to 1100 pp of ethanol. The heating conditions were once per second with a width of 5 m5ec and a voltage of 0.
．． A 7V pulse is added. In this example, the load resistance INl was set to IOKΩ, a detection voltage of 5 V was applied, and the output Vout was sampled immediately after the heating pulse ended. The response speed of the sensor 2 is suitable for practical use.

Table 1 shows the relationship between various heating conditions and characteristics.

Table - Month * Continuous heating 250 6.5 5.0 3
．． 0100m5ec/sec 800 13
7.0 3.550m5ec/see 600
8.0 8.0 4.015m5ec/sec
350 6.0 5.0 3.55m5ec
/see 300 4,0 4.5 4.5
15m5ec/l0sec 300 4.5 5
．． 0' 4.015m5ec/sin 400
3.0 G, 0 4.010m5e
c/min* oo IOMΩ IOMΩ I
MΩ* Gas concentration is 100 pp@ each, sensitivity is the ratio of resistance in air and gas, and in the last example 1/min.
Heating with 1 ossec pulse and measuring the resistance value just before the next heating. In this example, the resistance value in air cannot be measured and the resistance value in gas is displayed. The heating voltage is 0.7V in both cases, and the resistance value is displayed. Values are expressed in ohms.

According to the results shown in FIG. 4, FIG. 5, and Table 1, the gas sensor 2 operates in all cases, and no unsatisfactory results were obtained.

- Figure 9 shows the results when the width of the heating pulse was set to 0.2 m5ec. In this example, heater 4 is supplied with a width of 0 once per second.
, voltage 3. for 25 seconds. 'Add a 2V heating pulse,
The sensor output Vout was measured in air and gas.

Gas can be detected even under heating conditions of 0,2 m5ec/see. Note that the peak of the sensor output is 0 after pulse application.
, 4 m5ec, and 10m5 after pulse application.
After 4' elapsed, the sensor output decreased to almost 0.

The inventor's view regarding heating pulses is shown. The width of the pulse is preferably lμSee~2 sec, more preferably 50μsec~20 (lssec). Next, the pulse interval is preferably about IOOtasec~lOIain, more preferably 0,5 sec~2 mi.
Let it be n. Furthermore, the heating duty ratio is set to 115 or less,
Preferably it is 1/20 or less. The duty ratio is 9th
As is clear from the figure, it may be as small as 115,000 or less, for example, about 10'□@. Here, we will add some additional information regarding the lower limit of the pulse width. The pulse width itself may be extremely short, for example, 1 μsec. The limit of the pulse width is determined by the withstand voltage of the heater 4. As is clear from FIG. 9, shortening the pulse width causes a delay in the sensor output. This is probably due to a time delay between when power is applied to the heater 4 and when the metal oxide semiconductor layer 6 is heated. Then, in the case of a heating pulse of less than this time, the energy per pulse should be more important than the width or power of the heating pulse. Therefore, the width of the heating pulse may be made infinitely small as long as the circuit allows it and the heater 4 can withstand it.

6 to 8 show the gas sensor 2 used in the example. In Fig. 6, 4 is Fe-Cr-Al alloy or Ni-
A metal heating element such as a Cr alloy or a Pt wire, with a wire diameter of about 10 to 100 μm, is used here. 8
is a heat-resistant insulating coating provided on almost the entire surface of the metal heating element 4,
Alumina, silica, etc. are used. This coating 8 is provided by ion blasting, sputtering, plasma CVD, oxidation of a vacuum-deposited film such as A1 or Si, or coating with alumina sol, silica sol, or the like. The thickness of the coating 8 may be, for example, about 100 A to 5 μm.

6 is a metal oxide semiconductor layer such as 5nOt, In2O5, ZnO, etc. I O, + 2 is an electrode, made of Au or Pt
, nh, Ru5t, etc., and is provided by vacuum evaporation or application of a paste of these materials. When part of the heat-resistant insulating coating 8 is peeled off and the metal heating element 4 is used also as an electrode for the metal oxide semiconductor layer 6, only one electrode 10.12 may be used. In this case, it is preferable that the metal heating element 4 is made of a noble metal.

Reference numeral 14 is a heat-resistant insulating substrate made of alumina or the like, which may be made of stainless steel or other metal surface coated with glass or the like for insulation. Reference numeral 16 denotes a cavity, in which the metal compound solution and the substrate are formed when the metal oxide semiconductor layer 6 is formed by thermal decomposition of the metal compound solution, etc. This is to separate the two. In the example, a solution of an organic compound of Sn was thermally decomposed to form the metal oxide semiconductor layer 6. However, when the metal oxide semiconductor layer 6 is provided by sputtering, vacuum evaporation, etc., the cavity 16 is not necessary.

18.20.22 is film-like 11 [i1! printed with gold, Pd1Rust, etc. Therefore, it is better to provide it by vacuum evaporation or the like. Then, both ends of the metal heating element 4 are connected to the electrodes 18 and 22 by welding or the like, and the electrode IO is connected to the membrane electrode 18 and the detection electrode 12 is connected to the membrane electrode 20 using conductive adhesive such as gold paste or ruthenium oxide paste. Connect with agent 24. Instead of using the conductive adhesive 24, the membrane electrode 18.20 and the electrode 10.12 may be connected by ultrasonic heating or the like. 30 is a lead frame that also serves as an external pin, and each lead frame 30 and a membrane electrode +
Connect 8°20.22 with lead wire.

FIG. 7 shows an enlarged sectional view of the main parts of the sensor. In the figure, 26 is a welded portion between the metal heating element 4 and the membrane electrode 18°22. Then, the metal heating element 4 is welded to the membrane electrode 18, 22, and a conductive adhesive 24 is used. li pole 10.12
to 1-shaped electrode 18.20)? Continue. Note that the metal heating element 4
does not contact the substrate 14 at any other position. This is because the metal heating element 4 is slightly warped due to some play during welding.

The conductive adhesive 24 connects the electrode 10.12 and the membrane electrode 18.2.
0 has the effect of protecting the welded part of the metal heating element 4 from corrosion. This is because the structure of the metal heating element 4 changes due to welding and becomes susceptible to corrosion, so the conductive adhesive 24 protects the welded portion by shielding it from the atmosphere. If protection of the welded portion is not required, the conductive adhesive 24 may not be used for the membrane electrode 22. In the embodiment, the electrode 10 and the metal heating element 4 may be connected to the membrane electrode 18, or they may be separated and four membrane electrodes may be received on the substrate 14.

FIG. 8 shows the overall structure of the gas sensor 2. In the figure,
32 is a base made of synthetic resin or the like, and 34 is a cover made of synthetic resin or the like, which is formed into a cylindrical or prismatic shape.

In the two-leg example, a linear metal heating element 4 was used,
The metal heating element may be in the form of a foil, for example.

For the measurements shown in FIGS. 4 and 5 and Table 1, the sensor 2 was adjusted under the following conditions. A Fe-Cr-A1 alloy wire 2 with a diameter of 20 μm was coated with alumina sol (Kantal manufactured by Gabrius, Sweden, Kanthal is a trade name) and thermally decomposed at 800° C. to form an alumina film. This process was repeated 10 times to obtain an alumina coating 8 with a thickness of about 1 μm. Covering 8
was provided on the entire surface of the alloy wire 4. Alloy wire 4 with a length of 1.2 mm
It was cut into pieces and directly welded to the membrane electrodes 18 and 22. The insulating coating 8 at the welded portion 26 peeled off due to the pressure, welding current, etc. during welding, and welding could be performed without removing the coating 8 in advance.

After welding, put a mask on the alloy wire 2 and apply gold 7J1 by vacuum evaporation.
A pole 10.12 was provided. After forming the electrode 10.12, S
How many organic compounds of n, here S n (OCtl 5) 3 (
An isobutanol solution of 0(CH*)*N l-1*) was dropped and thermally decomposed at 500°C to obtain a metal oxide semiconductor layer 6 of S n Oy. Here, due to the cavity 16, the droplet and the substrate! Contact with 4 was prevented. Next, conductive adhesive 2
4 connected the electrodes 10.12 and also protected the welds of the electrodes 22. After these, the board! 4 to the lead frame 30, and electrodes +8.20.
22 was wire-bonded to the lead frame 30 to form the sensor 2.

[Effects of the Invention] According to the present invention, the power consumption of the gas sensor can be reduced.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of the embodiment, Fig. 2 is a circuit diagram of a modified example, and Fig. 3 is an operation waveform diagram of the embodiment. FIG. 2) shows the waveform of the sensor temperature, and FIG. 3) shows the waveform of the sensor output. FIG. 4 and FIG. 5 are characteristic diagrams of the embodiment. Figure 6 is a front view of the main parts of the gas sensor used in the example, Figure 7
The figure is an enlarged sectional view of a main part of the gas sensor used in the example, and FIG. 8 is a sectional view of the gas sensor used in the example. FIG. 9 is a characteristic diagram of the example. In the figure, 2 gas sensor, 4 metal heating element, 6 metal oxide semiconductor layer, 8 heat-resistant insulating coating. Patent applicant Figaro Giken Co., Ltd. Figure 7 Figure 8 Figure 5 Figure 6

Claims (4)

[Claims] (1) A heat-resistant insulating coating is applied to the surface of the metal heating element, a metal oxide semiconductor layer and at least one electrode are provided on the coating, and gas is detected from the resistance value of the metal oxide semiconductor layer. Using a heated gas sensor, the metal heating element is intermittently heated by heating pulses with a width of 2 seconds or less and a duty ratio of 1/5 or less, and the gas sensor is left near room temperature during the period when no heating pulses are applied. A gas detection method that detects gas based on the resistance value of a semiconductor layer. (2) A heat-resistant insulating coating is provided on the surface of the metal heating element, a metal oxide semiconductor layer and at least one electrode are provided on the coating, and gas is detected from the resistance value of the metal oxide semiconductor layer. A heating pulse with a width of 2 seconds or less and a duty ratio of 1/5 or less is applied to the metal heating element to intermittently heat the gas sensor, and the gas sensor is left near room temperature during the period when no heating pulse is applied. A gas detection device equipped with a pulse power source and a gas detection means for detecting gas from the resistance value of a metal oxide semiconductor layer during heating. (3) The gas detection device according to claim 2, characterized in that the pulse width of the pulse power source is 0.2 seconds or less, and the interval between pulses is 0.5 seconds to 10 minutes. gas detection device. (4) A heat-resistant insulating coating is provided on the surface of the metal heating element, a metal oxide semiconductor layer and at least one electrode are provided on the coating, and gas is detected from the resistance value of the metal oxide semiconductor layer. A heating pulse with a width of 2 seconds or less and a duty ratio of 1/5 or less is applied to the metal heating element to intermittently heat the gas sensor, and the gas sensor is left near room temperature during the period when no heating pulse is applied. A gas detection device equipped with a pulse power source and a gas detection means for detecting gas from the resistance value of a metal oxide semiconductor layer when left at room temperature.