JPH1183776A - Gas detector and its design method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily decide a combination of signals used by using a signal of a gas sensor having high sensitivity to gas to be detected as a main signal, and using a signal having different sensitivity to the gas and similar characteristics as a correction signal. SOLUTION: A pair of heaters h1 , h2 are disposed at both ends of metal oxide semiconductor 2 of a gas sensor S. And, temperature of the semiconductor 2 is periodically changed by altering a duty ratio of transistors T1 , T2 . That is, signals of a plurality of the gas sensors are outputted by temperature change, combined and hence gas to be detected is detected. For example, when outputs of the sensors S are sampled at a plurality of time points, a plurality of signals of the sensors are obtained. And, the signal of the sensor having high sensitivity to the gas is used as a main signal. Of signals of the sensor having high correlation to the main signal, sensitivities to the gas are different. And, the signal having similar characteristics to the signal to be corrected is used as a correction signal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas detector using a metal oxide semiconductor gas sensor and its design.

[0002]

2. Description of the Related Art As a metal oxide semiconductor gas sensor using a temperature change, there is an SnO2 type CO sensor TGS203 or the like (TGS203 is a trade name of FIGARO GIKEN). This gas sensor operates at a cycle of 150 seconds, assigning the first 60 seconds to the high temperature area and the next 90 seconds to the low temperature area, the final temperature in the high temperature area is 300 ° C, and the final temperature in the low temperature area is 80 ° C.
CO is detected from the resistance value of the metal oxide semiconductor at the end of the low temperature range. The resistance value of the sensor is almost inversely proportional to the CO concentration, and the relative sensitivity between hydrogen and CO is 1:10.
0 ppm and 100 ppm of CO are equivalent. Further, the initial distribution of the resistance value is 1 to 10 KΩ in 100 ppm of CO.

[0003] However, the metal oxide semiconductor gas sensor has a characteristic drift associated with use. In the case of this sensor, the resistance value increases at most twice in about two months from the start of use, and in several years thereafter, the resistance value increases. It decreases to about 1/2 of the initial value. Since the resistance value of the TGS 203 is almost inversely proportional to the CO concentration, the above drift is caused when the CO detection concentration is twice or more.
It means to change by 1/2 times.

[0004] The inventor has studied the use of the TGS 203 to combine signals at a plurality of time points accompanying a temperature change to correct drift. As in the conventional method, a signal in a low temperature range was used as a detection signal of CO, and a signal to be combined with the signal was searched. For this purpose, a search method for determining which signal should be used is required. For example, the TGS 203 operates in a cycle of 150 seconds. If sensor signals are sampled every three seconds, the total number of signals is 50, and there are extremely many combinations of these signals. And if there are many signals, it is extremely difficult to organize them.

Here, related prior art is shown. It has been proposed by Yoshikawa et al. To change the temperature of a gas sensor and regard the behavior of its resistance value as a temperature waveform and to perform a Fourier transform on this to detect gas (Analytical Chemistry VoL68, No. 13, 2067-2072
1996). In addition, there are many studies on combining a high-temperature signal and a low-temperature signal of a gas sensor (for example, US Pat. Nos. 4,896,143 and 4,399,684).

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to make it possible to easily determine a combination of signals to be used when detecting a gas using a temperature change of a metal oxide semiconductor gas sensor.

[0007]

According to the present invention, a plurality of gas sensor signals are extracted by changing the temperature of a metal oxide semiconductor gas sensor, and an object gas is detected by combining these signals. For example, if you sample the output of the gas sensor at multiple times,
A plurality of gas sensor signals are obtained. If a Fourier transform or the like is applied to the sampled output, components in the Fourier transform or the like become a plurality of gas sensor signals.

According to the present invention, a gas sensor signal having high sensitivity to a target gas is used as a main signal. Among the gas sensor signals having a high correlation with the main signal, the sensitivity to the target gas is different and the characteristic for the correction target is different. Are used as correction signals.

According to the present invention, a plurality of gas sensor signals are extracted by changing the temperature of the metal oxide semiconductor gas sensor.
In the design of a gas detection device that is configured to detect a target gas by combining these, a gas sensor signal having high sensitivity to the target gas is used as a main signal, and a gas sensor signal having a high correlation with the main signal is searched and searched. In the gas sensor signal, a signal having different sensitivity to a target gas and having similar characteristics to a correction target is used as a correction signal.

Preferably, as the correlation, a waveform of a gas sensor signal (hereinafter referred to as a temperature waveform) with respect to a temperature change is measured using a plurality of gas sensors, for example, about 10 to 100 gas sensors, and a correlation of a resistance value in a target gas is measured. Is used. Preferably, as the correlation, a temperature waveform is measured using a plurality of gas sensors, for example, about 10 to 100 gas sensors, and a correlation of a behavior of a resistance value in a correction target is used.

[0011]

According to the present invention, the temperature of the metal oxide semiconductor gas sensor is changed to detect gases such as CO, water vapor, ammonia, ethanol, and benzene. The temperature change is performed, for example, by applying a square wave power to the heater of the gas sensor, but may be a sine wave, a sawtooth wave, or the like. The type of the metal oxide semiconductor may be In2O3, ZnO, or the like in addition to SnO2 shown in the embodiment, and the structure of the sensor is arbitrary.

The main signal is a signal having high sensitivity to the gas to be detected, and the correction signal is a signal for correcting drift of sensor characteristics, temperature / humidity dependency, sensitivity to coexisting gas, and the like. In the present invention, a signal having a high correlation with the main signal with respect to the characteristic of the correction target and having a different sensitivity to the target gas is used as a correction signal.

Here, the inventor has discovered one rule of thumb.
For example, a drift of the gas sensor, that is, a change in sensor characteristics due to use is set as a correction target. The selection of the main signal can be made simply from the viewpoint of sensitivity to the target gas. Here, for example, several tens of sensors are used to measure the temperature waveform in the target gas or the like at the start of measurement, for example, about three days to one week after the start of use of the gas sensor. Here, a time point having a high correlation between the main signal and the resistance value is selected. This selection was made with respect to the initial characteristics at the start of use and does not reflect drift. However, if a signal having a high resistance value correlation is used here, a high correlation can be obtained even with respect to drift.

That is, two signals having a high correlation on a certain measurement day have similar behaviors with respect to other items, and if one of them is a main signal and the other is a correction signal, they are also used for drift correction. Get used. As a matter of course,
For example, when a drift is to be corrected, a drift-related behavior, for example, a change from the initial value of the resistance value may be measured, and a correlation for the change may be obtained to determine a correction signal. The inventor's knowledge here is that the two signals whose resistance values are initially correlated strongly correlate with the change in resistance value due to drift.

When converting to a generating function such as Fourier transform,
A component having high sensitivity to the target gas is defined as a main signal, and a component having a strong correlation with the sensitivity, a low sensitivity to the target gas, and a similar behavior in the correction target is defined as a correction signal.

According to the present invention, the correction signal to be combined with the main signal can be easily determined. Hereinafter, an embodiment in which a correction signal is selected using a correlation function and the validity of the correction signal selected by the cluster analysis will be confirmed. The structure of the gas detection device, its adjustment method, the algorithm of gas detection, etc. are also shown, because the structure itself of the gas detection device is required to be new, and the conventional adjustment is not enough for the adjustment method. . In the embodiment, a specific gas sensor, a specific detection target and a correction target are shown, but the present invention is not limited to these.

[0017]

【Example】

[0018]

[Structure of Gas Detector] FIG. 1 to FIG. 36 show an embodiment.
FIG. 1 shows the structure of the developed gas detecting device. S is a metal oxide semiconductor gas sensor, in which a TGS203 is used and a pair of heaters h1 and h2 are arranged at both ends of a SnO2 based metal oxide semiconductor 2. It is. The type, structure, material, and detection target gas of the sensor S are arbitrary. 4 is DC 5
The gas detection device is driven by the output VDD using a DC power supply such as V. In order to drive the pair of heaters h1 and h2 of the gas sensor S together, transistors T1 and T2 are used, and these are turned on / off at the same time. And the transistor T
When both T1 and T2 are turned on, current flows through the heaters h1 and h2, and the temperature of the metal oxide semiconductor 2 is periodically changed by changing the on duty ratio of the transistors T1 and T2. Here, in accordance with the operating conditions of the TGS 203, the high-temperature range is set to 60 seconds and the low-temperature range is set to 90 seconds. The waveform of the heater power is a square wave that changes in two stages, a high-temperature range and a low-temperature range. The final temperature in the low temperature range is 80 ° C. In the embodiment, the time is displayed as
The 0th second immediately before the end of the low temperature range is defined as the 0th second, the 0th to 60th second is defined as the high temperature range, and the 90th to 150th second (the 150th second is equal to the 0th second) is defined as the low temperature range. However, the heater waveform is not limited to a square wave, but may be a sine wave or a sawtooth wave.

A resistance ladder 5 is connected to the metal oxide semiconductor 2, and R1 to Rn are individual resistors. Here, it is assumed that each of the resistors R1 to Rn changes four times, for example, 0.5KΩ, 2KΩ, 8KΩ, 32KΩ, 128KΩ,
Six resistors of 512 KΩ are used. The accuracy of the fixed resistor is ±
An AD conversion error of about 2% is easily obtained, and an AD conversion error based on the switching of the resistance value is about ± 2%. When the transistors T1 and T2 are turned off, the power supply output VDD (hereinafter, referred to as a detection voltage Vc) flows to the resistance ladder 5 via the metal oxide semiconductor 2, and the output voltage to the resistance ladder 5 is A / D converted and processed.

Reference numeral 8 denotes a microcomputer.
Assume a one-chip microcomputer of bits. 1
Reference numeral 0 denotes the bus, reference numeral 12 denotes, for example, an 8-bit AD converter, reference numeral 14 denotes a resistor ladder control unit, which grounds only one of the resistors R1 to Rn, and uses the grounded resistor as a load resistor. As described above, the output voltage to the resistor ladder is AD
AD conversion is performed by the converter 12. It is natural that the output voltage to the resistance ladder 5 may be further divided and A / D-converted.
The same is true even if the voltage on the side is AD-converted. Reference numeral 16 denotes a heater control unit that controls on / off of the transistors T1 and T2 to generate a temperature cycle including a high-temperature region for 60 seconds and a low-temperature region for 90 seconds. Reference numeral 18 denotes an EEPROM control unit.
0 is an EEPROM.

FIG. 3 shows the structure of the EEPROM 20.
For example, here, CO is a detection target, and the detection range is CO5.
The range is about 10 times of 0 to 600 ppm. As the reference signal, three points of 65 ppm, 200 ppm, and 400 ppm of CO are used, and the logarithm LnR0,
The logarithm LnR6 of the sensor resistance at the 6th second and the logarithm LnR69 of the sensor resistance at the 69th second (the initial stage of the low temperature range) are used. Ln represents a natural logarithm, and a subscript such as 0 of R0 represents a timing based on 0 seconds. Similarly, at 200 ppm and 400 ppm of CO, the three reference signals at the 0th, 6th and 69th seconds are stored in the form of the logarithm of the sensor resistance. Cards 51 to 53 are used when the reference signal for each density is considered as one card. In addition, the card 54 is used to record the usage history of the CO detection device. That is, a record of the total use time and the past CO warning is stored as the elapsed time. The total usage time is a cumulative value of the time during which the power supply of the CO detection device is turned on. For example, the time unit is one day, and the cumulative usage time is stored in the card 54. As a record of the alarm, the date is stored each time a buzzer described later sounds.
As the date, a date based on the same standard as the total use time is stored. In this way, the day when the buzzer sounds is known.

Reference numeral 22 denotes an input / output, to which an adjustment switch 23 and a reset switch 24 are connected.
Is turned on, the EEPROM control unit 18
This switch is used only when adjusting the CO detection device. Reset switch 24
Is a switch for stopping the buzzer 38 from sounding.

The microcomputer 8 has a 4-bit arithmetic and logic operation unit 26, a sequence control unit 28 for operating the CO detection device at a period of 150 seconds, and the sequence control unit 28 has a built-in timer. . 3
Reference numeral 0 denotes a RAM, which is used as a volatile memory, and its configuration is shown in FIG. In the RAM 30, LnR0, LnR6, LnR6
Nine measurement data and reference signals at two concentrations are stored. As the reference signal, 65 ppm and 200 ppm on the low concentration side are always used, and when the gas concentration exceeds 200 ppm, the 65 ppm reference signal is replaced with a 400 ppm reference signal. When the gas concentration drops to 200 ppm or less, the reference signal of 400 ppm is replaced with the reference signal of 65 ppm. The gas detection range is 50-600 ppm and 5
The range of 0 to 65 ppm is close to the reference signal 65 ppm. Also 4
The range of 00 to 600 ppm is 1.5 times the range of 400 ppm of the reference signal, and the gas concentration can be accurately obtained using the 400 ppm reference signal. Excluding these ranges, when CO is generated, the gas concentration is determined by interpolation between the two reference signals using the reference signals on both sides of the actual CO concentration.

The RAM 30 also stores the obtained CO concentration, COHb (CO hemoglobin concentration in blood) converted from the CO concentration, and other auxiliary signals (for example, time data for configuring a timer on a daily basis). And so on.

Returning to FIG. 1, reference numeral 32 denotes an alarm control unit which operates the LEDs 39 and 40 via a drive circuit 36 to thereby control the blood CO 2.
The buzzer 38 sounds when the hemoglobin concentration is, for example, 5% or more. When the buzzer 38 sounds, the EEPROM control unit 18 writes the date of the alarm on the card 54. Reference numeral 34 denotes a program memory in which various constants and other data used for temperature correction are recorded. These data are common fixed data even when the sensor S changes. All data for each sensor is recorded in the EEPROM 20. 42 is a thermistor for measuring the ambient temperature, and 44 is a temperature / humidity correction unit.

[0026]

[Sampling and Logarithmic Conversion by Detector] FIG. 4 shows an average temperature waveform of ten sensors. If the sampling point used in the embodiment is indicated by a circle in the waveform of CO 100 ppm, sampling is performed at 150 seconds, 6 seconds, and 69 seconds. The resistance value of the sensor changes about 10 times in the range of 30 ppm to 300 ppm of CO, and the resistance value changes about 10 times between 0 second and 69 seconds. If a variation in sensor resistance, a change in ambient temperature and humidity, and the like are added to the above, the range of the AD conversion is about 0.5 to 500 KΩ in resistance value. So in this range A
The resistances R1 to Rn are set to 0.5 KΩ or more so that D conversion can be performed.
The output voltage VRl is changed to a resistance ladder immediately before each sampling timing, and the load resistance is switched according to the value. A of VRl
The D conversion itself can be performed within one second, and it is sufficient to determine which resistor is used at each sampling point according to the value at that time.

FIG. 5 is an enlarged view of the initial temperature waveform in the high temperature region for another 10 sensors. Atmosphere is 0 ° C, relative humidity 96%, 20 ° C 65%, 50 ° C
40%, the range of ± 2δ (δ is the standard deviation) and the average value are shown. Although the gas concentration is 100 ppm CO, the resistance value changes slightly less than 10 times at each timing due to fluctuations in ambient temperature and humidity. Further, the resistance values at the 0th and 6th seconds are substantially equal. For example, the same load resistance as at the 0th second may be used at the 6th second. However, preferably, for example, a resistance value of 0 second (or 149 second to ensure sampling before shifting to a high temperature region) is determined from a signal of 148 seconds, and a resistance value of 6 seconds from a resistance value of 5 seconds is determined. Determine the load resistance of
Similarly, the load resistance at 69 seconds is determined from the resistance value at 68 seconds.

FIG. 6 shows a sampling algorithm. When the time reaches 148 seconds, the output voltage is AD-converted, and this value is 1/3 to 2 of the detection voltage Vc (same as VDD).
Confirm that it is within the range of / 3. In this range, the ratio of the resistance value between the sensor resistance and the load resistance is in the range of 2: 1 to 1: 2. If the output voltage is correct, the output voltage is kept as it is. If the output voltage is not correct, the load resistance is switched so that the output voltage falls within this range. Next, when it reaches the 0th second, the output voltage is AD-converted, and the output voltage VRl obtained by AD conversion is used to obtain 0 according to the equation (1).
Find the logarithm of the sensor resistance at the second. Similarly, at 5 seconds, it is checked whether the value of the load resistance is correct, and the logarithm of the sensor resistance at 6 seconds is obtained. Further, it is checked whether the value of the load resistance is correct at the 68th second, and the logarithm of the sensor resistance is obtained at the 69th second.

LnR = 2−4VRl / Vc + LnRl (1) When the logarithm of the sensor resistance is approximated to the first-order term as in equation (1), R / Rl is 1 and the error is 0, and R / Rl is 1 / 2 or 2, the error is 2%, and R / Rl is 1/3 or 3, the error is 11%. In the embodiment, since the purpose is to detect the CO concentration with an error of ± 20% or less, the error of ± 10% is too large. Therefore, the resistance ladder 5 is controlled so that the ratio between the sensor resistance and the load resistance is maintained in the range of 2 to 1/2 at three points of 0 second, 6 seconds, and 69 seconds.

The conversion from VR1 to the logarithm of the sensor resistance according to the equation (1) is a linear conversion, which is a very simple conversion. However, six load resistors were required accordingly. In order to reduce the number of load resistors to, for example, four, the range of R / Rl is maintained in the range of 4 to 1/4, more preferably in the range of route 8 to 1 / route 8. For this purpose, conversion up to the third order term is necessary. When the logarithm of the sensor resistance is series-expanded by VRl, there are no second-order terms, and equations (2) and (3) take into account the third-order terms. When equations (2) and (3) are used,
When R / Rl is 1, the conversion error is 0%, when R / Rl is 1/4 or 4, the conversion error is 4%, and when Rl is 1/3 or 3, the conversion error is 2%. Therefore, for example, the values of the resistors R1 to Rn are set to 16
It is changed by a factor of two, more preferably by a factor of eight or nine. For example, when the values of the resistors R1 to Rn are 1KΩ and 8K
Ω, 64 KΩ, and 512 KΩ. In this way, the range of 0.5 to 1 MΩ can be converted to logarithm with an error of 2% or less. LnR = 2x + 2/3 × x 3 + LnRl (2) x = 1-2VRl / Vc (3)

[0031]

[Adjustment of Gas Detection Device] FIG. 7 shows a procedure for adjusting the gas detection device of FIG. At this time, the adjustment switch 23 is turned on to enable the writing of the reference signal to the EEPROM 20. In the following description, it is assumed that the CO detection device is set in the adjustment tank, and after the detection device is set, the power is turned on to operate. Then, for example, 65 ppm of CO is injected. Then, the microcomputer 8 generates LnR0, LnR6, and LnR69 for writing into the RAM 30. This is written on the card 51 of the EEPROM 20. The same procedure is then followed, increasing the CO concentration to 200 ppm.
Further, the CO concentration is increased to 400 ppm. If the CO concentration is increased in a predetermined procedure as described above, the EEPROM
20 can be written with a reference signal. As a result, there is no need to adjust the variable resistance to store the reference signal, and the adjustment operation is simplified.

Here, the CO detector is set in the adjustment tank, but only the sensor S may be set. Then, the resistance value of the sensor S may be A / D converted by, for example, an AD converter of about 12 bits, recorded in a personal computer or the like, and written in the EEPROM 20.
In this case, the sensor S is not incorporated in the CO detection device, but the sensor S is handled as a set with the EEPROM 20, and these are assembled into a separately assembled CO detection device. The parts other than the sensor S and the EEPROM 20 can be handled in exactly the same manner as a normal electronic circuit, and even a manufacturer having no experience with a gas sensor can assemble a CO detection device.

[0033]

[Correlation of gas sensor signal] FIG.
(Surface temperature of metal oxide semiconductor measured by thermistor,
(1 sensor number).

FIGS. 9 to 12 show T in CO at 100 ppm.
The correlation of the resistance value of the sensor signal of GS203 is shown. The energization period is about 40 days after two times of leaving. The resistance value of the sensor was measured at 3 points at 50 points in a 150 second cycle. The number of TGS 203 used was 10, and FIG.
10 are randomly extracted data out of 20 non-defective products in the sample used in (1). 9 shows the correlation of the resistance value with the 0th second, FIG. 10 shows the correlation of the resistance value with the 6th second, and FIG.
Shows the correlation of the resistance value with the 60th second, and FIG. 12 shows the correlation of the resistance value with the 69th second. 13 to 15 show the correlation of the resistance value between the 0th second value and other timings for the same 10 samples. FIG. 13 shows the result in 300 ppm CO, FIG. 14 shows the result in 100 ppm CO + 300 ppm H2, and FIG.
It is in 0 ppm.

The correlation between the late stage of the low temperature region (120 seconds to 150 seconds) is strong (FIG. 9), and the early stage of the high temperature region (6 seconds) has a strong correlation from the latter stage of the low temperature region to the middle stage of the high temperature region (30 seconds). FIG.
0), the late stage (60 seconds) of the high temperature region is the middle stage (30 seconds) of the high temperature region.
Second), but the correlation with the latter part of the low temperature region is weak (FIG. 11). Further, the initial period (69 seconds) of the low temperature region correlates with 60 seconds or 90 seconds (FIG. 12). CO10 even in CO300ppm
Although the correlation tendency does not change from that in 0 ppm (FIG. 13),
The tendency of the correlation changes with the mixed gas of hydrogen and hydrogen (Fig. 1
4) The tendency of the correlation is different even in hydrogen (FIG. 15).

FIG. 1 shows the correlation between the change in the resistance value due to the drift.
6 is shown. The gas is 100 ppm of CO, and the correlation with the others is based on 0 second in 100 ppm of CO on the third day from the start of energization. On the third day from the start of energization, the resistance value at 0 seconds and the resistance value at 6 seconds have a strong correlation, but this correlation does not break down thereafter.
Since the correlation coefficient of the 0 second resistance value is almost 1 even if the measurement date is changed, the 6 second resistance value on the third day of the energization start is 0%.
It strongly correlates with the resistance value at the second. Accordingly, the drift of the resistance value at the 0th second thereafter can be corrected by the resistance value at the 6th second on the third day of energization.

FIG. 17 shows the criterion for determining the correction signal when the correlation of the resistance value is used, and FIG. 18 shows the criterion for determining the correction signal when the correlation for the change in the resistance value due to the drift is used. . In FIG. 17, the resistance value at the 0th second is used as a main signal for CO detection in accordance with the conventional use of the TGS 203. Energization start 3
At an appropriate day such as the day or the seventh day, at each timing t, the resistance value in CO is measured, and the average and variance of the resistance value are obtained for each timing. Next, the other correlation C of the resistance value between the 0th and tth seconds is obtained, and a signal having a strong correlation, a low CO sensitivity, and a timing similar in sensitivity to hydrogen as an interfering gas is used as a correction signal. I do.

The same applies to the processing in FIG. 18, and the resistance value at the 0th second is used as the main signal for CO detection in accordance with the conventional method of using the TGS 203. For example, over a period of about 40 days to one year, CO
The average value and the variance of the ratio of the resistance value to the initial value (day 3 or day 7) at the same timing are determined. Next, the other correlation C of the resistance value between the 0th and tth seconds is obtained, and a signal having a strong correlation, a low CO sensitivity, and a timing similar in sensitivity to hydrogen as an interfering gas is used as a correction signal. I do.

[0039]

[Drift of Gas Sensor Signal] FIGS. 19 to 24 show drift characteristics of the sensor resistance. The samples used were 45, good (20), and samples left for more than 2 years (8
Pcs), defective products (7 pcs), and samples (10 pcs) collected after being assembled in the CO alarm. In each figure, the horizontal axis indicates the sensor resistance at 0 second on a logarithmic scale, and the vertical axes indicate the 3rd second (FIG. 19), the 6th second (FIG. 20), and the 12th second (FIG. 2).
1), 30 seconds (FIG. 22), 60 seconds (FIG. 23), 12
The sensor resistance at the 0th second (FIG. 24) is similarly shown on a logarithmic scale. The horizontal axis 1 is a reference signal in CO 100 ppm at 0 second (third day of energization start), and the vertical axis is 1 is a reference signal at 100 ppm CO at 6 seconds and the like (third day of energization start).
It is. 19 to 24 are normalized by the reference signal on the third day from the start of energization in 100 ppm of CO.

Each point in the figure indicates a measurement point associated with 5 weeks of energization. If 45 TGS 203 are used for 5 weeks, the resistance of the sensor will be about twice as high and low. In FIG. 20, the increase in resistance is concentrated on a narrow straight line with a slope of 1 in a two-dimensional phase space of 6 seconds and the like and 0 seconds. This axis is called the drift axis. The reason why the drift axis is not clear at 30 ppm or 300 ppm of CO is due to the dispersion of the concentration dependency of TGS203. That is, the concentration dependency is not uniform and CO3
Since the initial points in 0 ppm and 300 ppm are not aligned at one point, the drift axis is unclear due to dispersion of the initial points. A straight line connecting three points of 30 ppm, 100 ppm, and 300 ppm of CO will be referred to as a concentration axis. The initial characteristics of the TGS 203 are on this concentration axis, and the concentration axis moves parallel to the direction of the drift axis with use. FIG.
9 to FIG. 24, the resistance of a part of the sample has started to be lowered, which indicates that the coordinate (phase point) in 100 ppm of CO exists in the fourth quadrant lower left from the origin. The drift for lowering the resistance proceeds along the drift axis as in the case of the higher resistance.

In the combination of the signal at the 3rd second of the valley of the resistance value and the signal at the 0th second (FIG. 19), the drift axis and the concentration axis are close to each other, and it is difficult to correct the drift. This is because the signal at the third second (more than 100 ° C.) has similar CO sensitivity to the signal in the low temperature range (around 80 ° C.).

FIG. 21 also shows a drift axis similar to that of FIG. 20, but the distribution of the phase points around the drift axis is widened.
This indicates that the correlation between the drift of the signal at 0 seconds and the drift of the signal at 12 seconds is weaker than that at 0-6 seconds. In the characteristics at the 30th to 0th seconds in FIG. 22, the distribution around the drift axis is wider,
It is difficult to distinguish from 0 ppm. 23 in FIG.
Among the characteristics at −0 second, some of the ones that drifted the most at 300 ppm of CO tend to almost overlap the reference point at 100 ppm of CO. 24th second-0 in FIG.
In the characteristics of the second, since the signal at the second and the signal at the 120th are very similar, both the drift axis and the concentration axis are common,
And all the phase points are gathered around one straight line.

From these facts, it can be used for drift correction that the initial signal in the high temperature range is, for example, 4 to 2
A signal at the 0th second, preferably a signal at the 5th to 15th seconds.
The other party to be combined is a signal in the latter half of the low temperature range, for example, a signal at the 90th to 150th seconds, preferably at the 120th to 150th seconds. In any of FIGS. 19 to 23, the concentration axis and the drift axis are oblique, and two axes corresponding to the CO concentration and the drift cannot be obtained in the orthogonal coordinate system. If a concentration axis orthogonal to the drift axis can be obtained, it means that there is an axis that is not affected by the drift, and the coordinates on that axis are determined only by the gas concentration. However, no such axis could be found.

[0044]

[Negative hydrogen sensitivity] In each figure, the behavior of a mixed gas of 100 ppm of CO and 300 ppm of hydrogen and the behavior in 1000 ppm of hydrogen are also shown. As is clear from FIG. 20, the sensitivity is slightly negative for hydrogen. For example, when each point of 100 ppm of CO + 300 ppm of hydrogen in FIG. 20 is translated along the drift axis and the intersection with the concentration axis is obtained, the obtained concentration range is 80 ppm to 60 ppm of CO. On the other hand, CO1
The distribution of each point in 00 ppm for 5 weeks is narrow, and when the point of intersection with the concentration axis is determined by translating along the drift axis, the distribution range is about 80 to 120 ppm of CO. The reason why the sensitivity to hydrogen is negative is that the signal at 6 seconds has a higher hydrogen sensitivity than the signal at 0 seconds. Therefore, in order to correct this, a phase space consisting of signals at the 0th and 69th seconds is used.

FIG. 25 shows the same energization data for 5 weeks in this case. As is clear from FIG. 25, when hydrogen is generated, the resistance value at the 69th second is significantly reduced, and is located at a place extremely far from the concentration axis. Therefore, a signal representing the hydrogen concentration is defined as the distance from the concentration axis in the downward direction in FIG.

Such a hydrogen detection signal is not accurate, and FIG. 25 does not use an oblique coordinate system. However, since it is used for correcting a small negative hydrogen sensitivity, even a hydrogen detection signal having no quantitative property can be used. Then, in the correction of the hydrogen sensitivity, the hydrogen sensitivity slightly negative in FIG. 20 is returned to 0, that is, a CO detection device which is extremely selective only for CO is designed, or the CO characteristic is changed like the original characteristic of the TGS 203.
Correcting the relative sensitivity of hydrogen to 10: 1 can be considered. Which of these to choose depends on the CO
This is a matter of the design policy of the detection device.

[0047]

[Temperature and Humidity Dependence] FIG. 26 shows the relative humidity of 96% at 20 ° C., 40% at 65 ° C.
% Dependence on temperature and humidity in CO30ppm, 100ppm, 30ppm
This is for 0 ppm. 20 ° C or 5 at 0 ° C
A phase point exists on a line different from 0 ° C., and a decrease in absolute humidity can be detected. As is well known, the TGS 203 mainly depends on the absolute humidity.

FIG. 27 shows another ten TGS 203 (less than 2 months from the start of use) having a resistance value between 0 ° C., relative humidity of about 96% and 20 ° C. relative humidity of 65% in 100 ppm CO. Shows the ratio. The horizontal axis represents the timing of the temperature change. Most of the temperature-humidity dependence is corrected as a side effect of drift correction at the 0th and 6th seconds.

FIG. 28 shows the same 10 TGSs 203 (FIG. 2).
5) in 100 ppm CO.
It shows the ratio of resistance values between 0 ° C. and about 40% relative humidity and 20 ° C. and 65% relative humidity. Most of the temperature-humidity dependence is corrected as a side effect of drift correction at the 0th and 6th seconds.

FIGS. 27 and 28 show that the temperature / humidity dependency is small in the initial stage of the high temperature range, and there is a problem that the temperature / humidity dependency is smaller than that in the low temperature range. In order to increase the temperature / humidity dependence in the early high temperature range for temperature / humidity correction, for example, a signal at 10 to 15 seconds is preferable, and the drift is accurately corrected using a signal at around 6 seconds. , 10-1
It is a matter of trade-off whether to emphasize the correction of the temperature and humidity dependency using the signal at the 5th second.

[0051]

[Information dimension of gas sensor] The number of independent signals included in the gas sensor will be referred to as information dimension. The information dimension is a concept clearly defined in the information science on the basis of entropy, but here, the number of independent signals is simply qualitatively determined and called the information dimension. As described above, the CO concentration and the degree of drift can be known from the combination of the signal at the 0th second and the signal at the 6th second, and the H2 concentration can be determined from the combination of the signal at the 0th second and the signal at the 69th second. I was able to know.

Using the temperature waveform of TGS203, C
Four signals of O, drift, hydrogen, and absolute humidity (FIG. 26) can be obtained. On the other hand, from the correlation of the temperature waveforms of the TGS 203 (FIGS. 9 to 16), it is known that the temperature waveforms can be classified into four stages: a late stage of a low temperature region, an early stage of a high temperature region, a late stage of a high temperature region, and an early stage of a low temperature region. The characteristics of the TGS 203 are affected by four factors: CO, hydrogen, absolute humidity, and drift.
Person. From these facts, it can be said that the temperature waveform of the TGS 203 includes four signals of CO, hydrogen, absolute humidity, and drift, and the information dimension of the temperature waveform of the TGS 203 is four-dimensional.

These four-dimensional information-dimensional signals are: 1) a main signal for detecting CO, for example, a signal for 90 seconds to 150 seconds, more preferably 120 seconds to 150 seconds, and 2) a signal for drift correction. For example, a signal for 4 to 20 seconds, more preferably 5 to 15 seconds, 3) a signal for detecting hydrogen in the early low temperature range, and 4) a combination of a signal in the early high temperature range and a late signal in the high temperature range without thermistor. It can be used as four kinds of signals, for correction of absolute humidity.

The fact that the information dimension of the TGS 203 is four-dimensional is supported by correlation analysis and cluster analysis.
The correlation analysis indicates that the signal in the high temperature range should be used as a signal for drift correction. The signal at the early stage of the high temperature region has a strong correlation with the signal at the late stage of the low temperature region, which is the main signal, and has a weak correlation with the signal at the latter stage of the high temperature region. In addition, the signal at the early stage of the low-temperature region and the signal at the latter stage of the low-temperature region have a weak correlation, and the signal at the early stage of the low-temperature region is selective to hydrogen and does not become a signal for drift correction.

The waveform data in FIG. 4 indicates that the CO sensitivity is very similar to that at the 0th second, for example, at the 3rd second at the beginning of the high temperature range, and thereafter the hydrogen sensitivity increases. Therefore, the signal for drift correction is limited to a signal of, for example, 4 to 20 seconds, preferably 5 to 15 seconds. That is, the characteristics of the gas to be detected are too similar at the third second, and the difference of the hydrogen sensitivity becomes remarkable after more than 20 seconds. This is supported by the cluster analysis in FIGS. 19 to 22, and it is found that there is an optimum point around the 6th to 12th seconds.

[0056]

[Drift correction] FIG. 29 shows the principle of drift correction.
The solid line in the figure is the concentration axis, and the broken line is the drift axis. And 6
The reference signal at three points of 5 ppm, 200 ppm and 400 ppm is E
It is recorded in the EPROM 20. By measurement, LnR0
And a point (a, b) on the phase space in two dimensions of LnR6. The coordinates of each reference signal in this phase space are shown in FIG.
Determined as 9 Then, the object is translated from the point (a, b) along the drift axis, and the coordinates of the intersection with the concentration axis are (e,
f). Once the coordinates (e, f) are obtained, the CO concentration can be obtained from the position on the concentration axis. The movement from the coordinates (a, b) to the coordinates (e, f) is a projection onto the density axis parallel to the drift axis.

The projection method may be arbitrarily selected. For example, data indicating the CO concentration may be written in the phase space of FIG. 29 to form a two-dimensional map, and the CO concentration may be obtained from the position on the map. The fact that the map is coarse and there is no data directly corresponding to each coordinate may be processed by interpolation between each point of the map. Alternatively, each reference point (l, m), (e, f), (q,
From r), three lines parallel to the drift axis are drawn, and a correction value for the CO concentration is written on each line, and the correction value is set to 1 on the concentration axis. Then, a correction value is determined so as to correct the increase in resistance due to the drift. Then, the density axis is moved in parallel so as to pass through the measurement point, the intersection point between the two correction lines on both sides is obtained, and the correction value at each correction line is obtained and interpolated. The logarithm of the sensor resistance at the 0th second is corrected by the correction value thus obtained, and is converted into a CO concentration. In these modified examples, the restriction of the projection can be reflected on the correction value of the correction line and the value of the map, and a fine operation on the projection is easy.

[0058]

[Signal Processing] FIGS. 31 to 36 show the calculation of the CO concentration. FIG. 31 shows a main loop. First, three variables a, b, and c are defined from measurement data. Next, the CO concentration is determined by a temperature correction subroutine (FIG. 32), a drift correction subroutine (FIG. 33), and a hydrogen correction subroutine (FIG. 34). Finally, the blood CO hemoglobin concentration COHb is determined from the CO concentration. The initial value of COHb is set to 0 at the time of reset. This conversion itself is already known, and k2, k3, and k4 are constants. Here, k4 is a value corresponding to about 30 ppm of CO, which is equal to or less than the lower limit of detection, and detection is not performed when the CO concentration is 30 ppm or less.

[0059]

[Temperature Correction Subroutine] In the temperature correction subroutine of FIG. A reference table of correction constants T1, T2, and T3 for a, b, and c from the ambient temperature is prepared in the program memory 34, and is read and added to a, b, and c.

[0060]

[Drift Correction Subroutine] FIG. 33 shows a drift correction subroutine. The inclination of the drift axis is 1, and (e-
a) and (fb) are equal. Therefore, f = e + (b−
a) is established. Therefore, one of the two unknowns e and f can be deleted. Next, it is checked whether np is equal to or larger than ab. If this condition is not satisfied, the measurement point is 200pp
When the drift axis is extended from m, it is below the drift axis, and the detected concentration is 200 ppm or less. Next, the point (e,
f) internally divides a line segment defined by two reference signals of 65 ppm and 200 ppm. From this, e and f are 65 ppm or 2 ppm.
The coordinates n, p, q, and r of the reference signal at 00 ppm are constrained by one relational expression, and the coordinates e can be solved using these.

There is no limitation on the projection of the obtained e, and the projection is similarly performed at a point extremely distant from the density axis or near the density axis. The projection is symmetrical above and below the density axis. On the other hand, as the drift from the concentration axis to the high resistance side becomes remarkable, it is preferable to limit the projection and correct only a part of the drift. In addition, when drifting from the concentration axis to the low resistance side, it is preferable to make correction more conservative than drift to the high resistance side. Further, the inclination of the drift axis at about 30 ppm of CO is slightly larger than the inclination of the drift axis at 100 ppm or more, and it is preferable to change the inclination of the drift axis for each concentration. Also, 30 ppm of CO is harmless and is not included in the detection target, and there is no need to perform drift correction on CO in such a low concentration range. Therefore, as shown in FIG. 35, it is preferable to make the correction asymmetric above and below the density axis, and to partially correct the drift as the distance from the density axis increases.

When a map is used or a plurality of drift axes are provided for each CO concentration, the above processing can be performed by manipulating the data in the map or manipulating the inclination of the drift axis. However, in the embodiment, after e is obtained, the program memory 3
The above-described processing is performed using the two-dimensional lookup table stored in step S4.
The headings of this look-up table are (ea) and e, where (ea) is proportional to the distance from the concentration axis. The sign of (ea) is inverted above and below the density axis. The value of e indicates the CO concentration, and the processing of the low concentration area or the processing of the high concentration area is determined by the value of e. Therefore, if the value of e is updated from the look-up table in accordance with (ea) and e, the correction is conservative in an area that is asymmetrical above and below the density axis and large in distance from the density axis, and is corrected in a low density area. Can be modest. However, the processing corresponding to FIG. 35 may not be performed.

When the value e is finally found, 65 ppm and 2
The internal division ratio y of the line segment between 00 ppm is determined. When y is 0, the CO concentration is 200 ppm, and when y is 1, the CO concentration is 65 ppm. During this period, there is a change in the CO concentration about three times, and if this is solved as it is, a second-order or higher term is required in the series expansion of exp (y), so consider the midpoint between 65 ppm and 200 ppm. In the vicinity of 200 ppm, the series is developed based on the concentration of 200 ppm, and in the vicinity of 65 ppm, the series is developed based on the concentration of 65 ppm. In this way, exp (y) = 1 +
Even when approximating y, there is almost no approximation error. In this way, the CO concentration before the correction of the hydrogen concentration is determined.

When the obtained phase point is above the drift axis passing through 200 ppm of CO, the CO concentration is 200 pp.
m is exceeded. Therefore, in this case, the EEPROM 20 is accessed to read the reference signal of 400 ppm of CO. Hereinafter, the CO concentration is determined in the same manner. The processing in this case is CO6
This is the same as the process when two reference signals of 5 ppm and 200 ppm are used.
A reference signal of 0 ppm may be used.

The temperature / humidity dependency has a gas concentration dependency as shown in FIG. 36, and the temperature / humidity dependency differs between a low concentration region and a high concentration region. However, at the stage of the temperature / humidity correction subroutine, C
O concentration is unknown. Therefore, after temporarily calculating the CO concentration, a two-dimensional look-up table stored in the program memory 34 from the ambient temperature T and the temporarily determined CO concentration is used.
Re-correct the density. This is a method of first-order approximation ignoring the temperature-humidity dependence of the CO concentration, and re-correcting the temperature-humidity dependence of the CO concentration using the obtained temporary CO concentration. In the lookup table, the amount of increase or decrease in the CO concentration is stored using the provisional CO concentration and the ambient temperature as headings.
Obtain the O concentration. The processing corresponding to FIG. 35 can be omitted.

[0066]

[Hydrogen correction subroutine] When the CO concentration is determined, hydrogen correction is performed. FIG. 34 shows the processing, and FIG. 30 shows the principle thereof. In a two-dimensional phase space defined by the logarithm of the resistance value at 0 seconds and the logarithm of the resistance value at 69 seconds, the coordinates of the measurement point are (a,
c). This is 65ppm, 200ppm, 400
The point of intersection at the time of moving vertically upward in FIG. 30 to the ppm concentration axis is (a, g). Then, the difference between g and c is h, and h
The hydrogen concentration is determined by the In this case, it is determined whether or not it is necessary to use a signal of 400 ppm as a reference signal from whether or not the value of a exceeds n. If a is equal to or less than n, the EEPROM 20 is accessed to read the reference signal of 400 ppm. . Since the point (a, g) is on the line connecting the 200 ppm reference signal and the 400 ppm reference signal, one equation is generated for the coordinate g, and g
Can be requested. When g is obtained, h is obtained. For example, k1 × h is added to the CO concentration obtained in the main loop of FIG. 31, for example, using k1 as an appropriate positive constant. As a criterion for the addition, for example, the hydrogen concentration dependency of the CO detector is set to 0, or the relative sensitivity of CO to hydrogen is set to an appropriate value such as 10: 1. When a is larger than n, that is, when the coordinate point (a, c) obtained in FIG. 30 is on the right side of the reference signal of 200 ppm, the reference signals of 65 ppm and 200 ppm are used. Then, h is obtained in the same manner as described above, and the hydrogen concentration is corrected.

[0067]

[Fourier Transform] The embodiment has been described by taking a specific sensor as an example and a specific heater waveform as an example, but these are examples. In the embodiment, the temperature waveform of the gas sensor signal is used without being converted into the generating function. However, a signal subjected to Fourier transform or the like may be used. For example, Laplace transform or Fourier transform is performed on the temperature waveform of the gas sensor, and gas is detected by combining a plurality of converted components. In this case, a component having high sensitivity to the target gas may be used as the main signal, and a component having low sensitivity to the target gas and having a similar behavior in the correction target may be used as the correction signal.

[Brief description of the drawings]

FIG. 1 is a block diagram of a gas detection device according to an embodiment.

FIG. 2 is a diagram showing a configuration of a RAM in the gas detection device of the embodiment.

FIG. 3 is a diagram showing a configuration of an EEPROM in the gas detection device of the embodiment.

FIG. 4 is a characteristic diagram showing a waveform of a resistance value of the gas sensor used in the embodiment.

FIG. 5 is a characteristic diagram showing a resistance value waveform of a gas sensor used in an example in an initial high temperature range.

FIG. 6 is a flowchart showing a sampling algorithm in the gas detection device according to the embodiment.

FIG. 7 is a flowchart showing an adjustment algorithm in the gas detection device according to the embodiment.

FIG. 8 is a characteristic diagram showing a temperature change of the gas sensor used in the embodiment.

FIG. 9 is a characteristic diagram showing a correlation between a resistance value of a sensor signal at 0 seconds and a sensor signal at another time point on the same measurement date in the embodiment.

FIG. 10 is a characteristic diagram showing the correlation between the resistance value of the sensor signal at 6 seconds and the sensor signal at another time point in the example in 100 ppm of CO on the same measurement date.

FIG. 11 is a characteristic diagram showing a correlation between a resistance value of a sensor signal at 60 seconds and a sensor signal at another time point in 100 ppm of CO on the same measurement date in the embodiment.

FIG. 12 is a characteristic diagram showing a correlation between a resistance value of a sensor signal at 69 seconds and a sensor signal at another time point in 100 ppm of CO on the same measurement date in the embodiment.

FIG. 13 is a characteristic diagram showing a correlation between a resistance value of the sensor signal at 0 seconds and a sensor signal at another time point in the same measurement date in 300 ppm of CO in the embodiment.

FIG. 14 is a graph showing the relationship between the sensor signal at 6 seconds and the sensor signal at another time point in the embodiment, on the same measurement date, CO 100 ppm + H 2.
Characteristic diagram showing correlation of resistance value at 300 ppm

FIG. 15 is a characteristic diagram showing a correlation between a resistance value in H2 1000 ppm on the same measurement date between a sensor signal at 60 seconds and a sensor signal at another time point in the embodiment.

FIG. 16 is a characteristic diagram showing a correlation between resistance values at different measurement dates between a sensor signal at 69 seconds and a sensor signal at another time point in the embodiment.

FIG. 17 is a flowchart illustrating evaluation of correlation in the embodiment.

FIG. 18 is a flowchart showing an evaluation of a correlation in a modified example.

FIG. 19 is a characteristic diagram showing a drift characteristic on a 0-3 second plane in the example.

FIG. 20 is a characteristic diagram showing a drift characteristic on a 0-6 second plane in the example.

FIG. 21 is a characteristic diagram showing a drift characteristic on a 0-12 second plane in the example.

FIG. 22 is a characteristic diagram showing a drift characteristic on a 0-30 second plane in the example.

FIG. 23 is a characteristic diagram showing a drift characteristic in a 0-60 second plane in the example.

FIG. 24 is a characteristic diagram showing a drift characteristic on a 0-120 second plane in the example.

FIG. 25 is a characteristic diagram showing a drift characteristic on a 0-69 second plane in the example.

FIG. 26 is a characteristic diagram showing a temperature-humidity characteristic in a plane for 9-60 seconds in the example.

FIG. 27 is a characteristic diagram showing the temperature and humidity dependency between 20 ° C.-65% RH and 0 ° C. of the gas sensor.

FIG. 28: 20 ° C.-65% RH of gas sensor and 50 ° C.
Characteristic diagram showing temperature and humidity dependence between -40% RH

FIG. 29 is a characteristic diagram showing a mechanism for calculating the CO concentration in the embodiment.

FIG. 30 is a characteristic diagram showing hydrogen correction in the embodiment.

FIG. 31 is a flowchart showing a main program in the embodiment.

FIG. 32 is a flowchart illustrating temperature and humidity correction in the embodiment.

FIG. 33 is a flowchart illustrating drift correction in the embodiment.

FIG. 34 is a flowchart showing correction to coexisting hydrogen in the gas detection device according to the embodiment.

FIG. 35 is a characteristic diagram showing details of drift correction in the embodiment.

FIG. 36 is a characteristic diagram showing details of temperature and humidity correction in the embodiment.

[Explanation of symbols]

 2 Metal oxide semiconductor 4 DC power supply 5 Resistance ladder 8,48 Microcomputer 10 Bus 12 AD converter 14 Resistance ladder control unit 16 Heater control unit 18 EEPROM control unit 20 EEPROM 22 I / O 23 Adjustment switch 24 Reset switch 26 Arithmetic logic operation unit 28 Sequence control unit 30 RAM 32 Alarm control unit 34 Program memory 36 Drive circuit 38 Buzzer 39, 40 LED 51 to 54 Card 42 Thermistor 44 Temperature / humidity correction unit S Metal oxide semiconductor gas sensor h1, h2 Heater T1, T2 Transistors R1 to Rn resistance

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Kazuko Takamatsu 1-3-5 Senba Nishi, Minoh City Inside Figaro Giken Co., Ltd.

Claims (4)

[Claims] 1. A gas detection device which detects a target gas by combining a plurality of gas sensor signals by changing a temperature of a metal oxide semiconductor gas sensor, wherein a gas sensor signal having high sensitivity to the target gas is mainly used. Used as a signal, among the gas sensor signals having a high correlation with the main signal, a signal having a different sensitivity to the target gas and having a characteristic similar to the correction target is used as a correction signal,
Gas detector. 2. A gas detection device having a high sensitivity to a target gas in a gas detection device designed to detect a target gas by combining a plurality of gas sensor signals by changing a temperature of a metal oxide semiconductor gas sensor. Is used as a main signal, a gas sensor signal having a high correlation with the main signal is searched, and among the searched gas sensor signals, a signal having a different sensitivity to a target gas and having similar characteristics to a correction target is used as a correction signal. A method for designing a gas detection device, characterized in that it is used. 3. The method for designing a gas detection device according to claim 2, wherein a plurality of gas sensors are used as the correlation and a correlation between resistance values in a target gas is used. 4. The method according to claim 2, wherein a plurality of gas sensors are used as the correlation, and a correlation of a behavior of a resistance value of a correction target is used.