JP4248126B2 - Gas detection method and apparatus - Google Patents

Description

[0001]
[Field of the Invention]
The present invention relates to correction for reducing the gas concentration dependency of a gas sensor in a high concentration region.
[0002]
[Prior art]
It is known to detect a gas by heating a metal oxide semiconductor such as SnO2 with a heater and periodically changing the temperature of the metal oxide semiconductor. This technique is initially used for detecting CO, and is currently used for detecting a plurality of gases such as CO and methane, CO and LPG, and is also used for detecting ozone, chlorine, ammonia, and the like. The gas sensor temperature change pattern was initially heated in both high and low temperatures to bring the gas sensor to a temperature above room temperature. A thing consisting of leaving the gas sensor to room temperature has also been put into practical use. In the gas detection using the temperature change of the gas sensor, the concentration dependency of the gas sensor output decreases in the high concentration region, and therefore the gas detection in the high concentration region is not reliable.
[0003]
[Problems of the Invention]
A basic object of the present invention is to compensate for the decrease in concentration dependency of a gas sensor in a high concentration region and to increase detection accuracy ( claims 1 and 2 ).
An additional object of the present invention is to provide a specific method for compensating for a decrease in CO concentration dependency of a gas sensor in a high concentration region.
[0004]
[Structure of the invention]
The present invention detects a gas from a resistance value of a metal oxide semiconductor in a method of detecting CO while periodically changing the temperature of a metal oxide semiconductor whose resistance value varies depending on a gas between a high temperature region and a low temperature region. the timing for a first timing of the low temperature region than the first timing provided at least two early second timing low temperature range, mainly detect CO said resistance value at the first timing In the case where the CO concentration mainly obtained from the first timing is a predetermined value or more, CO is mainly detected from the second timing .
[0005]
The meaning of mainly the resistance value at the first timing and the resistance value at the second timing mainly means that the resistance value at the first timing and the resistance value at the second timing are For example, the specific gravity of the resistance value is increased, or the resistance value of the second timing is increased.
[0006]
Under the CO conditions of the present invention , the first timing is generally less dependent on relative humidity and less sensitive to miscellaneous gases such as hydrogen, but the concentration dependence is reduced at high concentrations of CO. At the second timing, the humidity dependency is large and the sensitivity to hydrogen is high, but the CO concentration dependency is large even in a high concentration range. Therefore, when the CO concentration mainly obtained from the first timing is a predetermined value or more, CO is mainly detected from the second timing.
[0007]
The present invention also provides a gas sensor in which a resistance value of the metal oxide semiconductor is changed by a gas, and the power applied to the heater of the gas sensor is periodically changed to increase the temperature of the metal oxide semiconductor. In the gas detection device that is changed between the low temperature range and the low temperature range, means for sampling the output of the gas sensor and detecting CO at the first timing in the low temperature range, and the CO concentration obtained at the first timing Means for detecting CO by sampling the output of the gas sensor at a second timing earlier than the first timing and at a lower temperature range than the first timing.
[0008]
[Operation and effect of the invention]
In the present invention, the detection target gas is CO, and the temperature change pattern of the metal oxide semiconductor is composed of a repetition of a high temperature region and a subsequent low temperature region, and both the first timing and the second timing are in the low temperature region. When it is detected that CO of a predetermined concentration or more exists at the timing of 1, CO is detected from the resistance value of the metal oxide semiconductor at the second timing in the low temperature region earlier than the first timing. In this way, it can be solved that the CO concentration dependency of the metal oxide semiconductor is reduced with respect to high concentration of CO.
[0010]
In the invention of claim 2, the temperature of the metal oxide semiconductor is periodically changed between a high temperature region and a low temperature region, and the CO output is detected by sampling the output of the gas sensor at the first timing in the low temperature region. When the CO concentration is greater than or equal to a predetermined value, CO is detected at a second timing that is earlier in the low temperature region than the first timing. In this way, the problem that the CO concentration dependency of the metal oxide semiconductor is reduced in a high concentration region can be compensated.
[0011]
【Example】
1 to 7 show an embodiment. FIG. 1 shows a circuit configuration of the embodiment. Reference numeral 2 denotes a gas sensor, RH denotes a heater resistance thereof, and RS denotes a resistance value of the metal oxide semiconductor of the gas sensor 2. A load resistor RL is connected in series to the metal oxide semiconductor, 4 is a power source such as a battery, 6 and 8 are a pair of switches, switch 6 is for turning on the heater, and switch 8 is a gas sensor. This is for applying a detection voltage.
[0012]
Reference numeral 10 denotes a microcomputer, reference numeral 12 denotes an AD converter, and reference numeral 14 denotes a heater control unit, which controls the heater by turning on / off the switch 6. A sampling control unit 16 turns on / off the switch 8 to control a detection voltage applied to the gas sensor, and drives the AD converter 12 when the switch 8 is on.
[0013]
Reference numeral 18 denotes a low concentration detection unit, which detects CO of 400 ppm or less, 20 is a high concentration detection unit, detects CO of 400 ppm or more, 22 is an alarm unit, and outputs the CO concentration and the like to the outside. The low concentration detection unit 18 has a conversion table, a function, and the like for converting the resistance value of the metal oxide semiconductor at the first timing into the CO concentration, and the high concentration detection unit 20 at the second timing. Conversion tables and functions for converting the resistance value of the metal oxide semiconductor into the CO concentration.
[0014]
In the example of the gas sensor 2 used in FIG. 2, 30 is an insulating substrate, 32 is a heat insulating glass film, 34 is a heater, 36 is an insulating film between layers, and 38 is a metal oxide semiconductor film. An SnO2 film (for example, a catalyst such as Pd with a thickness of 20 μm) was used. The type of the gas sensor itself is arbitrary, and for example, a bead-shaped metal oxide semiconductor with a coil-shaped heater combined electrode and a center electrode embedded therein may be used.
[0015]
The temperature change pattern of the gas sensor is arbitrary. In the embodiment, the heater is heated in pulses, the metal oxide semiconductor is heated in pulses, and the metal oxide semiconductor is left near room temperature for other periods. It was. In this case, the pulse heating period and immediately after that correspond to the high temperature range, and the other periods correspond to the low temperature range. Other than this, for example, the high temperature region may be about 300 ° C., the low temperature region may be about 100 ° C., and CO may be detected using the first timing and the second timing in the low temperature region. In order to generate a high temperature region or a low temperature region, in addition to driving the heater with a square wave (in the case of the embodiment), a triangular wave, a sine wave, or the like may be added to the heater.
[0016]
FIG. 3 shows an operation pattern of the gas sensor 2. The gas sensor 2 operates in a cycle of 1 second. The heater is turned on for the first 14 milliseconds to heat the metal oxide semiconductor to a little over 300 ° C., and then the heater is turned off for 986 milliseconds. Decreases to near room temperature before 100 milliseconds. The detection target gas is CO, the first timing is 998 ms, and the second timing is 255 ms. The period of the pulse heating is not limited to 1 second, for example, about 1 second to 3 minutes, and the width of the pulse heating is, for example, about 5 milliseconds to 10 seconds.
[0017]
4 and 5 show examples of waveforms of sensor resistance values in CO of 30 to 1000 ppm. This is the resistance value of one sensor. In FIGS. 4 and 5, each line represents CO 30 ppm, 70 ppm, 150 ppm, 400 ppm, and 1000 ppm from the top.
[0018]
As is apparent from FIG. 4, the resistance values of CO 400 ppm and CO 1000 ppm are close to each other after the 800 ms, and the CO concentration dependency at 400 ppm or more is almost lost. However, in the 1-second cycle drive, the humidity dependence of the sensor resistance decreases and the sensitivity to hydrogen decreases as the time after pulse heating elapses. Therefore, in the embodiment, the first timing is set to the 998 ms. And
[0019]
The second timing is at 255 msec, which is a timing at which the CO resistance dependence of the sensor resistance is large. As is apparent from FIG. 4, the CO concentration dependency is large in the range of 150 ms to 350 ms, and this range is preferable for the second timing. However, at this timing, the humidity dependency is large and the sensitivity to hydrogen is high. In the section where the heater is on from 0 second to 14 milliseconds in FIG. 5, there is a sensor resistance peak near the 40 milliseconds time, and CO concentration dependency appears after the peak, and CO concentration dependency As described above, the maximum is about 150 msec to 350 msec.
[0020]
FIG. 6 shows the measurement algorithm of the embodiment. When the gas sensor 2 is driven at a cycle of 1 second, the heater is turned on in a pulse for the first 14 msec, and the high concentration flag is set in the previous detection of CO at the first timing, at the 255 msec The sensor resistance sampling (second timing) is added, and the CO concentration is calculated from the relationship between the sensor resistance and the CO concentration obtained at 255 milliseconds. When the high density flag is not set, sampling at 255 msec is omitted.
[0021]
The 998 ms is the first timing, the sensor resistance is sampled, and the CO concentration is obtained. When the determined CO concentration is a predetermined value K (for example, 400 ppm) or more, a high concentration flag is set. When the concentration is 400 ppm or more, the CO concentration obtained at 255 ms is used, and otherwise, the CO concentration obtained at 998 ms is used.
[0022]
FIG. 7 shows the relationship between the true value of the CO concentration and the display value obtained for the five lots of the gas sensor 2. Each line in FIG. 7 represents five lots, and each lot has five sensors. FIG. 7 shows the average value. The broken line in FIG. 7 indicates the display value of the CO concentration obtained from the sensor resistance at 998 msec, and the solid line is the display value when 400 ppm or more is obtained by the sensor resistance at 255 msec with the algorithm of FIG.
[0023]
In the measurement of the data in FIG. 7, the relationship between the sensor resistance and the CO concentration at 255 msec and 998 msec is obtained for each gas sensor, and after aging for one month, the measurement is performed again to obtain the result of FIG. It was. When processing at the first timing (998 ms) even at a concentration of 400 ppm or more, only a display value of about 400 to 500 ppm is obtained for a CO concentration of 1000 ppm. On the other hand, if 400 ppm or more is processed with the sensor resistance at the second timing (255 msec), the CO concentration can be accurately obtained over a wide range of 30 to 1000 ppm.
[0024]
In the algorithm of FIG. 6, the CO concentration is obtained using 100% of the second timing signal when the displayed value is 400 ppm or more. However, the weight of the first timing signal and the second timing signal is calculated using CO of 400 ppm or more. It may be obtained by averaging or the like. For detection in the high density region, the specific gravity of the signal at the second timing is increased, and in the low concentration region, the specific gravity of the signal at the second timing is decreased. In addition, sampling itself is always performed at both the first timing and the second timing, and a signal at the second timing may be used when high concentration CO is detected.
[0025]
Although specific examples have been shown in the embodiments, the CO concentration dependency of the gas sensor in high concentration CO is not limited to the gas sensor in FIG. This also occurs when a sensor in which an electrode is embedded is used by changing the temperature. It also occurs when other gas sensors detect the CO in the low temperature range by changing the temperature between the high temperature range and the low temperature range. In any of these cases, the low-concentration side mainly processes the sensor resistance at the first timing, and the high-concentration side mainly processes the sensor resistance at the initial second timing at a lower temperature side than the second timing. Then, the CO concentration can be obtained more accurately. In the embodiment, the example is to compensate for the decrease in CO concentration dependency in the high concentration range, but such a phenomenon also occurs in methane, LPG, ammonia, ozone, chlorine, etc., and is a gas to be detected. The type of is not limited to CO.
[Brief description of the drawings]
FIG. 1 is a block diagram of an embodiment. FIG. 2 is a sectional view of a gas sensor used in the embodiment. FIG. 3 is a diagram showing an operation pattern of the embodiment. FIG. 5 is a partially enlarged view of FIG. 4. FIG. 6 is a flowchart showing the operation algorithm of the embodiment. FIG. 7 is a characteristic diagram showing the display value of the CO concentration in the embodiment and the conventional example.
2 Gas sensor 4 Power supply 6, 8 Switch 10 Microcomputer 12 AD converter 14 Heater control unit 16 Sampling control unit 18 Low concentration detection unit 20 High concentration detection unit 22 Alarm unit 30 Insulating substrate 32 Insulating glass film 34 Heater 36 Insulating film 38 Metal oxidation Semiconductor film

Claims (2)

In the method of detecting CO while periodically changing the temperature of a metal oxide semiconductor whose resistance value changes depending on gas between a high temperature region and a low temperature region ,
The timing for detecting the gas from the resistance value of the metal oxide semiconductor, a first timing of the low temperature region, provided at least two early second timing low temperature range than the first timing, the first CO is mainly detected from the resistance value at the timing,
A gas detection method comprising: mainly detecting CO at a second timing when the CO concentration obtained mainly from the first timing is a predetermined value or more . A metal oxide semiconductor whose resistance value changes by the gas, the gas sensor so as you heated by a heater, by periodically changing the electric power applied to the heater, the temperature of the metal oxide semiconductor, a high temperature region and low temperature region In the gas detection device adapted to change to
Means for sampling the output of the gas sensor to detect CO at a first timing in a low temperature range;
And means for detecting CO by sampling the output of the gas sensor at a second timing at an initial low temperature range than the first timing when the CO concentration obtained at the first timing is equal to or higher than a predetermined value. A gas detection device.

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