JP3750998B2 - Exhalation detection method and apparatus - Google Patents

Description

[0001]
[Field of the Invention]
The present invention relates to gas detection, and more particularly to detection of poisoning of a gas sensor.
[0002]
[Prior art]
Metal oxide semiconductor gas sensors are known to be poisoned by catalyst poisons such as silicone vapor. Japanese Patent Application Laid-Open No. 2001-194330 states that a resistance value in air increases in heat-cleaned gas sensors. Has been reported. However, according to the data of this publication, the alarm concentration distribution for CO at the time of detection of poisoning ranges from 4400 to 6300 ppm, and when the poisoning is detected, the gas sensor is extremely deteriorated. May cause CO poisoning.
[0003]
[Problems of the Invention]
A basic object of the present invention is to provide a more practical poisoning detection method and apparatus .
An additional problem with the present invention is to compensate for the lack of sensitivity during poisoning.
[0004]
[Structure of the invention]
According to the breath detection method of the present invention, after the metal oxide semiconductor gas sensor provided with the heater is heated, the degree of change in the resistance value of the metal oxide semiconductor in the process of shifting to a stable state is obtained. If the gas sensor poisoning is detected because it is small , and then the poisoning is not detected, the resistance value of the metal oxide semiconductor changes discontinuously beyond the first predetermined value, so that exhaled air is directed to the gas sensor. When the poisoning is detected, the resistance value of the metal oxide semiconductor changes discontinuously over a second predetermined value smaller than the first predetermined value. If it is detected that exhaled air has been blown and further poisoning has not been detected, the resistance value of the metal oxide semiconductor is compared with the detection threshold when the first time has elapsed since the detection of exhalation of breath. And let the exhaled gas When the poisoning is detected, the metal oxide semiconductor is compared with the detection threshold at the time when the second time longer than the first time has elapsed since the detection of the inhalation of the breath, and the gas in the breath is detected. It detects, and the said detection threshold value is changed so that the sensitivity fall by poisoning may be compensated.
[0005]
The breath detection device of the present invention includes a metal oxide semiconductor gas sensor provided with a heater, means for heating the metal oxide semiconductor by energizing the heater, and a metal oxide in the process of transitioning to a stable state after heating. When the degree of change in the resistance value of the semiconductor is determined and the degree of change is small, poisoning detection means for detecting poisoning of the gas sensor, and when the poisoning detection means does not detect poisoning After the detection of the presence or absence of poisoning, the resistance value of the metal oxide semiconductor changes discontinuously beyond the first predetermined value, so that it was detected that exhaled air was blown toward the gas sensor and the poisoning was detected In this case, after the detection of the presence or absence of poisoning, the breath is blown because the resistance value of the metal oxide semiconductor changes discontinuously over a second predetermined value smaller than the first predetermined value. Blow detection to detect If no poisoning was detected, the resistance value of the metal oxide semiconductor at the time when the first time has passed since the detection of exhalation was compared with the detection threshold value to detect the gas in the exhalation. When poisoning is detected, the gas in the expired gas is detected by comparing the metal oxide semiconductor with the detection threshold at the time when the second time longer than the first time has elapsed since the detection of the inhalation of the breath. And a means for changing the detection threshold so as to compensate for the sensitivity reduction due to poisoning.
[0006]
Preferably, a reference value is obtained from the resistance value of the metal oxide semiconductor and stored, the reference value is converted into a detection threshold value, and compared with the resistance value of the metal oxide semiconductor. The gas is detected below the detection threshold, and the conversion condition from the reference value to the detection threshold is changed so as to compensate for the sensitivity reduction due to poisoning when poisoning is detected .
[0007]
Further, for example, when the heating is heat cleaning of the metal oxide semiconductor by energizing the heater, the degree of decrease in the resistance value after the heat cleaning is small, so that poisoning can be detected .
[0008]
In this specification, pulse heating means, for example, the case where the heating time is 2 seconds or less, the pulse heating time is preferably 1 second or less, and in particular, the pulse heating time is 1 second or less, and pulse heating and pulse heating are performed. The non-heating time is 50 times, preferably 100 times or more of the pulse heating time. In the heat cleaning, the gas sensor is heated even in cases other than the heat cleaning, and is performed, for example, irregularly in response to a predetermined event such as when the power is turned on. The heat cleaning time is, for example, 3 seconds or more, and in the heat cleaning, heating is performed at a temperature higher than the heating temperature at other heating timings.
[0009]
[Operation and effect of the invention]
The inventors have found that in a poisoned gas sensor, the change in resistance value during transition to a stable state after heating to a high temperature is small compared to a normal sensor. In pulse heating, the resistance value increases during the transition to a stable state after pulse heating due to poisoning, and the increase in resistance value decreases due to poisoning (FIG. 10). In heat cleaning, the degree of decrease in resistance value during the transition to a stable state after heat cleaning is reduced by poisoning (FIG. 5). In this invention, poisoning of the gas sensor can be detected .
[0010]
【Example】
1 to 6 show an embodiment of a portable breath odor detection device as an example. The gas sensor 2 used in FIG. 1 shows a substrate 4 such as alumina, 6 a heat insulating film using glass, 8 a film heater, 10 an insulating film using a glass film or a silica film. In the figure, 12 is a film-like metal oxide semiconductor, and here, a SnO2 film having a thickness of 20 μm was used. The substrate 4 is die-bonded to a rigid resin base with an adhesive or the like, but may be suspended in the air and supported by a lead wire. The shape, structure, and material of the gas sensor are arbitrary. For example, a gas sensor in which a detection electrode is arranged at the center of a coil-shaped heater / electrode and is embedded in a bead shape with a metal oxide semiconductor such as SnO 2 may be used.
[0011]
FIG. 2 shows the configuration of the gas detection apparatus. Reference numeral 20 denotes a battery power supply, which is a 3V power supply of single alkaline batteries × 2, 22 is a switch, and 24 is a load resistance. Reference numeral 26 denotes a microcomputer, reference numeral 28 denotes a sampling unit which includes an AD converter, and reference numeral 30 denotes a heater control unit which turns on the heater 8 via the switch 22. A timer 32 generates a timing signal necessary for turning on / off the heater, sampling the resistance value (sensor resistance) of the metal oxide semiconductor 12, and detecting poisoning.
[0012]
Reference numeral 34 denotes a poisoning detection unit that detects the presence or absence of poisoning and the degree thereof from the change in sensor resistance in a predetermined time zone after the end of heat cleaning. Here, the poisoning detection is performed based on the ratio of the sensor resistance at the beginning and end of the predetermined time zone, and in addition to this, the sensor resistance value itself at the end of the predetermined time zone may be taken into account. Reference numeral 36 denotes a rise detection unit, which detects that exhaled air has been blown because the sensor resistance changes discontinuously. Reference numeral 38 denotes a background contamination detection unit, which detects that the heat cleaning is insufficient or the background is contaminated from the sensor resistance at a predetermined time after the completion of the heat cleaning. Reference numeral 40 denotes a gas detection unit that uses, for example, the ratio of the sensor resistance at the time of rising detection and the sensor resistance after the elapse of a predetermined time, or adds the sensor resistance at the time of rising detection to this ratio. Detect gas concentration.
[0013]
42 is an I / O which displays the degree of bad breath etc. via the LEDs 44 to 46, and if the gas sensor 2 is poisoned and cannot be detected, this is indicated, and the background is contaminated and cannot be detected. If so, this is displayed. A start processing unit 48 starts the microcomputer 26 when the switch 50 is turned on, and stops the microcomputer 26 under predetermined conditions. Note that an appropriate switch may be provided between the metal oxide semiconductor 12 and the power supply 20 in order to prevent a detection current from flowing through the metal oxide semiconductor 12 when the gas detection device is not used.
[0014]
FIG. 3 schematically shows the driving pattern of the gas sensor. When the gas detector is turned on, for example, the heater 8 is energized continuously for 6 seconds to heat-clean the metal oxide semiconductor, and then the heater is turned on, for example, every 8 milliseconds, for example, every 250 milliseconds. After a predetermined time (120 milliseconds) has elapsed, a sampling pulse is generated and the sensor resistance is read as indicated by a broken line.
[0015]
4 to 6 show the operation algorithm and characteristics of the embodiment. When the switch is turned on and the power is turned on, for example, heat cleaning is performed for 6 seconds, sensor resistance 1 second after the end of heat cleaning (7 seconds), and sensor resistance 4 seconds after the end of heat cleaning (10 seconds) The presence / absence of poisoning and its level are detected by the ratio. When the poisoning is detected, if the poisoning is extremely significant, it is displayed that the detection is impossible due to the poisoning via the LED or the like. If the degree of poisoning is not extreme, the subsequent detection conditions are changed according to the degree of poisoning.
[0016]
Although it may be detected by a pressure sensor or the like that exhaled air has been blown, the rise is detected from a discontinuous change in sensor resistance. For example, when there is no poisoning, it is assumed that the sensor resistance has decreased by 3 LSB or more in 250 msec, and when the poisoning is detected, if there is a change of 2 LSB or more in 250 msec, exhalation is assumed. Then, the sensor resistance at the time of rising detection is stored as a reference value in the gas detection unit 40 of FIG. Here, the reference value is extremely low because of insufficient heat cleaning or background contamination. Therefore, for example, when the sensor resistance at the time of rising detection is lower than a predetermined reference value, the determination threshold is changed as a function of the reference value. The determination threshold is a constant for multiplying the reference value, and the detection threshold is obtained by converting the reference value using the determination threshold. In changing the judgment threshold value, the coefficient to be multiplied by the judgment threshold value may be changed discontinuously, for example, in three steps according to the reference value, or the constant may be changed continuously according to the reference value. May be.
[0017]
The sampling of the output during exhalation shifts the sampling timing depending on the presence or absence of poisoning. When there is no poisoning, the response to exhalation is fast. For example, a sensor signal 3 seconds after the rising edge detection is used, and when there is poisoning, a sensor signal 5 seconds after the rising edge detection is used. Note that the sampling time shift from the state of no poisoning may be changed in a plurality of stages depending on the degree of poisoning. The degree of exhalation contamination is detected based on the ratio between the sensor output during expiration and the reference value. This represents the degree of bad breath when the gas sensor is a bad breath sensor, and the alcohol concentration when it is an alcohol sensor. For example, three detection threshold values are provided in the dirt determination, and the degree of dirt is detected in four stages by comparison with these.
[0018]
For generation of the detection threshold, for example, three types of reference constants are prepared, and this is multiplied by a constant for changing the determination threshold at the time of background contamination and a constant depending on the degree of poisoning. If the poisoning is significant or the background is contaminated, the constant is controlled so that exhaled air is contaminated even if the change from the reference value is small. In this way, the degree of bad breath and the degree of alcohol are identified in four stages, and the results are displayed on the LEDs 44 to 46, and then heat-cleaned for about 2 seconds, for example, and the power is turned off.
[0019]
FIG. 5 shows the sensor resistance waveforms at the time of poisoning and at the normal time, and these are the average values of the waveforms of the five sensor resistances. The sensor was poisoned by exposing the sensor to a 10 ppm hexamethyldisiloxane atmosphere for one day. The sensor resistance during heat cleaning does not differ greatly between normal products and poisoned products, and there is no significant difference in sensor resistance immediately after the end of heat cleaning. The change in sensor resistance in 3 seconds from the 7th to 10th time is small for poisoned products and large for normal products. Note that the temperature of the metal oxide semiconductor at 7 seconds is about 100 ° C., and the temperature of the metal oxide semiconductor 12 at 10 seconds is room temperature. Therefore, the degree of poisoning can be detected by using the ratio of the resistance values between the 10th and 7th seconds, or the resistance values of the 10th and 7th seconds added to them.
[0020]
When poisoning is detected, the condition for detecting the rise of exhalation is changed. This is because the responsiveness to the water vapor in the exhaled breath decreases due to poisoning. If poisoning occurs, the response to exhalation decreases, so the sampling time of the output during exhalation is extended, for example, by 2 seconds. Also, since sensitivity to exhaled methyl mercaptan also decreases, the constant used to generate the detection threshold is changed. Thus, the influence of poisoning can be corrected, and the methyl mercaptan concentration, alcohol concentration, etc. in exhaled breath can be measured almost accurately.
[0021]
FIG. 6 shows the relationship between the degree of poisoning and the sensitivity to 1 ppm methyl mercaptan. This figure shows measured values for 17 gas sensors. The group on the lower left side of the figure is a gas sensor that is not poisoned, and the group on the center to the upper right is a group of gas sensors that are poisoned. The poisoning coefficient on the horizontal axis is the ratio of the sensor resistance between the 10th and 7th seconds, and this value approaches 1 due to poisoning. The vertical axis represents the ratio of resistance values between the 12th and 15th seconds, and the sensitivity is lost due to poisoning, and this value also approaches 1. It can be seen that the poisoning coefficient and methyl mercaptan sensitivity correlate well, and it is possible to evaluate the presence or absence of poisoning by the poisoning coefficient and to accurately correct the decrease in methyl mercaptan sensitivity.
[0022]
Reference Example FIGS. 7 to 11 show a reference example using poison detection of a CO detection device as an example . When the circuit configuration is shown in FIG. 7, the gas sensor 2 used is that of FIG. 1, and the same reference numerals as those in FIG. Reference numeral 23 denotes a switch for connecting the metal oxide semiconductor 12 to the battery power source 20, and reference numeral 60 denotes a new microcomputer. 62 is a new timer, and 64 is a new sampling unit equipped with an AD converter, which controls the switch 23 and reads the output voltage to the load resistor 24. A new heater control unit 66 turns on the switch 22 by the heater signal H to apply power to the heater 8. Reference numeral 68 denotes a CO detection unit, and reference numeral 70 denotes a poisoning detection unit, which statistically measures the rate of increase in sensor resistance after pulse heating and detects poisoning. Reference numeral 72 denotes a new I / O, which generates a CO detection signal when CO is generated, and generates a poison detection signal when poisoning is detected, and notifies the generation of CO and the deterioration (poisoning) of the gas sensor 2 by the notification unit 74.
[0023]
FIG. 8 shows a driving pattern of the gas sensor 2. The operation cycle is, for example, 60 seconds, and during the first 14 milliseconds, the heater 8 is turned on to heat the metal oxide semiconductor 12 in a pulsed manner, and the maximum temperature during the pulse heating is about 400 ° C. Then, for example, the sensor signal at the first second from the beginning of the cycle is read by the sampling unit, and CO is detected. For detection of poisoning, the sensor resistance increase rate after pulse heating (hereinafter simply referred to as pulse heating) by the heater 8 is used. Here, sensor signals at 15 msec and 30 msec from the beginning of the cycle are used. In this embodiment, the time is expressed with 0 ms as the beginning of the cycle. For example, the end of the pulse heating is 14 ms, and the sampling time of the poisoning detection signal is 15 ms and 30 ms.
[0024]
An acceleration test was conducted to degrade the gas sensor 2. Ten gas sensors 2 were used in an atmosphere having a relative humidity of 95% at 50 ° C., and the change in the alarm concentration with respect to CO was examined by taking it out from the constant temperature and humidity chamber. The initial value of the alarm concentration for CO is 100 ppm. Of the 10 gas sensors, the most severely deteriorated, the CO alarm concentration (calculated from the CO concentration dependency of the resistance value of the gas sensor) increased from 100 ppm to 300 ppm on the 46th day at 50 ° C. and 95% relative humidity. On the 56th day, the alarm concentration of CO reached 1000 ppm. In the remaining nine gas sensors, the alarm concentration on the 56th day was 400 ppm or less.
[0025]
A sensor deteriorated in an atmosphere at 50 ° C. and a relative humidity of 95% is considered to have been poisoned by water vapor, and is hereinafter referred to as a poisoned sensor or a poisoned product. 9 and 10 show waveforms of resistance values of the poisoning sensor and another normal sensor (one piece) immediately after being taken out from the constant temperature and humidity chamber. The waveform of the resistance value in the air (20 ° C. relative humidity 65%) is shown for the poison sensor, and the waveform in the air, 1000 ppm of hydrogen and 100 ppm of CO is shown for the normal sensor. FIG. 9 shows a waveform at time 0 to 20 seconds, and FIG. 10 shows a waveform at time 0 to 100 msec.
[0026]
In the waveform of the poisoning sensor, the resistance value during pulse heating is also high, and the increase rate of the resistance value after the end of pulse heating is slight. However, the resistance value during pulse heating increases by about 10 to 100 times from the air in the normal state in the case of a normal sensor in an atmosphere (in air) having an extremely low temperature such as −10 ° C. and a low absolute humidity. It is difficult to detect the presence or absence of poisoning from the resistance value itself during heating. On the other hand, the rate of increase in resistance value after pulse heating rather increases in low temperature and low humidity, so if the rate of increase in resistance value after pulse heating is used, in low temperature and low humidity (atmosphere with low absolute humidity), It is not mistakenly judged as poisoning.
[0027]
Next, in 1000 ppm of hydrogen, the rate of increase in resistance after pulse heating is negligible, but this is done by checking the sensor resistance at 30 msec or 15 msec (checking that the resistance value is low). Can be distinguished from poison. In addition, in the humid season such as the rainy season, the rate of increase in sensor resistance after pulse heating decreases. However, in the case of a normal product, even in the high humidity period, the resistance increase rate is sufficiently larger than that of the poisoning sensor (the resistance value increases about 10 to 20 times between 15 msec and 30 msec). The sensor resistance at 15 msec (normally about 10 KΩ) is sufficiently lower than the sensor resistance at 15 msec (about 1 MΩ) during poisoning. For these reasons, it is possible to detect the presence or absence of poisoning by using the sensor resistance ratio between the 15 msec and the 30 msec. In order to increase the reliability of detection, the sensor resistance value itself at the 15 msec or 30 msec may be taken into account. The 15 msec and 30 msec are examples of appropriate times for detecting a transient resistance value increase after pulse heating.
[0028]
FIG. 11 shows a poisoning detection algorithm in the reference example . Although the gas sensor operates at a cycle of 60 seconds, poisoning is detected every 7 hours because the poisoning detection time constant is set to about one week to 30 days and signals at various times during this period are used. .
[0029]
When the poisoning detection subroutine is activated, the sensor resistance Rs15 at the 15 msec and the sensor resistance Rs30 at the 30 msec are stored, it is checked whether the ratio of Rs30 and Rs15 is less than 10, and Rs15 Is checked whether it is 50 kΩ or more. In the example of FIG. 10, the ratio of Rs30 and Rs15 is about 3 for poisoned products and about 40 for normal products. This ratio also increases in a dry atmosphere. Although this ratio decreases in an atmosphere of hydrogen or in a humid atmosphere, Rs15 rarely exceeds 50 kΩ. The check of the value of Rs15 may be omitted or may be changed to a check of Rs30.
[0030]
Since it is dangerous to judge poisoning with a single sign of poisoning, the detection result of poisoning is statistically calculated. The principle of statistics is to obtain a moving average of the values of Rs30 / Rs15, and determine that the poisoning is caused when the moving average becomes 10 or less, for example. Here, when the ratio of Rs30 and Rs15 is 10 or less and Rs15 is 50 kΩ or less, the counter is incremented by one. However, the maximum value of the counter is 255. If any of the above conditions is not satisfied, the counter is decremented by 1 assuming that there is no sign of poisoning, and the minimum value of the counter is set to 0. Next, it is checked whether or not the counter value is 32 or more, and it is notified that the gas sensor has been poisoned when the counter value is 32 or more. The counter value is fixed so that the result is not canceled. When the value of the counter is less than 32, the poisoning notification is not performed because there is no poisoning.
[0031]
Since the presence / absence of poisoning is determined every 7 hours and the counter value is set to 32, the minimum time from the occurrence of poisoning to the notification of poisoning is about 9 days. This period can be appropriately changed within a range of, for example, about 1 week to 30 days. Once the poisoning is notified, the counter value is fixed because it is not considered as a normal sensor even if the poisoning sign disappears thereafter. However, the counter value for notifying poisoning is set to 32 or more, and the counter value for canceling poisoning notification is set to less than 16, for example, to provide a margin for detection and cancellation of poisoning and notify poisoning. You may allow the counter value to increase or decrease later. This is effective when the patient is temporarily poisoned but can recover from it later.
[0032]
In addition, although Example 1 (FIGS. 1-5) did not show the statistics of poisoning detection, you may statisticalize with the same algorithm as FIG.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a gas sensor used in an embodiment. FIG. 2 is a block diagram of a gas detection apparatus in the embodiment. FIG. 3 is a diagram showing a driving pattern of the gas sensor in the embodiment. FIG. 5 is a flowchart showing an operation algorithm of the apparatus. FIG. 5 shows poisoning detection, rise detection by inhalation of breath, change of sampling timing of output during expiration due to poisoning, and change of determination threshold. Characteristic diagram [Fig. 6] Characteristic diagram showing the correlation between the detection result of poisoning in the example and sensitivity to 1 ppm methyl mercaptan [Fig. 7] Block diagram of the gas detector of the reference example [Fig. 8] In the reference example FIG. 9 shows the output waveform of the sensor for 20 seconds in the drive pattern of FIG. 8, showing the behavior of the resistance value of the poison sensor in the air, In 1000ppm of hydrogen The behavior of the resistance value in 100 ppm of CO is shown.
FIG. 10 is an enlarged view showing the behavior of the resistance value of the sensor in a section of 0 to 100 milliseconds in FIG. 9. FIG. 11 is a flowchart showing an algorithm for poisoning detection in a reference example .
2 Gas sensor 4 Substrate 6 Heat insulation film 8 Heater 10 Insulation film 12 Metal oxide semiconductor 20 Battery power source 22, 23 Switch 24 Load resistance 26, 60 Microcomputer 28, 64 Sampling unit 30, 66 Heater control unit 32, 62 Timer 34, 70 Poison detection unit 36 Rise detection unit 38 Background contamination detection unit 40 Gas detection unit 42, 72 I / O
44-46 LED
48 Start processing unit 50 Switch 68 CO detection unit 74 Notification unit

Claims (2)

After heating the metal oxide semiconductor gas sensor equipped with a heater, the degree of change in the resistance value of the metal oxide semiconductor in the process of transitioning to a stable state is obtained. detected,
Next, if poisoning is not detected, the resistance value of the metal oxide semiconductor changes discontinuously beyond the first predetermined value, so that it is detected that exhaled air has been blown toward the gas sensor, and If the poison is detected, the second predetermined value smaller than the first predetermined value is detected, and the resistance value of the metal oxide semiconductor changes discontinuously to detect that exhalation has been blown,
Furthermore, if no poisoning is detected, the gas in the exhalation is detected by comparing the resistance value of the metal oxide semiconductor with the detection threshold at the time when the first time has passed since the detection of inhalation of exhalation, When poisoning is detected, the gas in the expired gas is detected by comparing the metal oxide semiconductor with the detection threshold at the time when the second time longer than the first time has elapsed since the detection of the inhalation of the breath. And the breath detection method characterized by changing the said detection threshold value so that the sensitivity fall by poisoning may be compensated. Metal oxide semiconductor gas sensor provided with a heater, means for energizing the heater to heat the metal oxide semiconductor, and degree of change in resistance value of the metal oxide semiconductor during the transition to a stable state after heating And poisoning detection means for detecting the poisoning of the gas sensor because the degree of the change is small ,
If the poisoning detection means does not detect poisoning, the resistance value of the metal oxide semiconductor changes discontinuously beyond the first predetermined value after detecting the presence or absence of poisoning. When the poisoning is detected, the resistance value of the metal oxide semiconductor is equal to or more than a second predetermined value smaller than the first predetermined value after detecting the presence or absence of poisoning. Inhalation detecting means for detecting that exhaled air has been infused because of the discontinuous change,
If poisoning is not detected, the gas in the breath is detected by comparing the resistance value of the metal oxide semiconductor with the detection threshold at the time when the first time has passed since the detection of breath inhalation. And detecting the gas in the expiration by comparing the metal oxide semiconductor with the detection threshold at the time when the second time longer than the first time has elapsed since the detection of the inhalation of the expiration, and A breath detection apparatus, comprising: means for changing the detection threshold so as to compensate for a decrease in sensitivity due to poisoning.

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