JP2008267810A - Gas detector and gas detection method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gas detector and gas detection method capable of sufficiently correcting the variation of the sensor sensitivity by secular variation of a contact combustion type gas sensor. <P>SOLUTION: The gas detector includes: a concentration output acquisition means 11a for acquiring measurement concentration output indicating gas concentration to be detected measured by the contact combustion type gas sensor 41 and air base concentration output indicating the air base concentration; a storage means 12; a time amount acquisition means 11b for acquiring the time amount based on the air base concentration output and the relationship between the time amount existing in the storage means 12 and the air base concentration output when the difference between the measured concentration output and air base concentration output is a predetermined constant value; a sensor sensitivity acquisition means 11c for determining the sensor sensitivity based on the time amount and the relationship of the time amount existing in the storage means 12 and the sensor sensitivity; and a detection concentration acquisition means 11d for acquiring the detection concentration of the gas to be detected by correcting the difference between the measurement concentration output and air base concentration output using the sensor sensitivity. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a gas detection device and a gas detection method using a catalytic combustion type gas sensor, and relates to correction for secular change of the catalytic combustion type gas sensor.

  A conventionally known catalytic combustion type gas sensor has a sensitive element part and a compensating element part, and burns the gas to be detected by the catalytic action of the sensitive element part, and captures this combustion heat as a change in the resistance value of the platinum coil. It is configured. Among gases to be detected, gases with high polarity and large adsorption power, such as toluene, acetic acid, ethanol, etc., are adsorbed when the gas molecules are adsorbed on the catalyst surface of the sensitive element when driven at low temperatures and when driven at high temperatures. Burns instantaneously and the catalytic combustion reaction occurs simultaneously, so that the sensor output has a peak waveform that reaches a peak in a short time and then gradually decreases. On the other hand, a nonpolar or small polarity gas such as methane, hydrogen, or carbon monoxide has a small adsorption power, so the above phenomenon does not occur, and the sensor output gradually increases until it stabilizes at a steady value.

  As described above, by utilizing a specific peak waveform in a specific type of gas such as toluene, it is possible not only to detect the gas concentration but also to classify the gas type using a catalytic combustion type gas sensor. Such a catalytic combustion type gas sensor that utilizes an adsorption phenomenon of a specific type of gas is also called an adsorption combustion type gas sensor.

  On the other hand, the gas sensor undergoes secular change, and the sensor output gradually changes. In order to correct such a secular change, various methods are proposed as seen in, for example, Japanese Patent Laid-Open No. 2006-194776 and Japanese Patent Laid-Open No. 7-270315.

  The gas chromatograph apparatus disclosed in Japanese Patent Application Laid-Open No. 2006-194776 includes a detector 30 including a semiconductor gas sensor provided on the exhaust side of a gas separation column, and an arithmetic processing unit 33. In the calibration mode, the arithmetic processing unit 33 outputs so that the gas concentration of the detection target gas component obtained from the peak of the output of the detector 30 matches the gas concentration of the calibration gas input using the input unit 38. A calibration calculation formula is corrected, and in the detection mode, the peak of the output of the detector 30 is corrected using the calibration calculation formula for all the detection target gas components, and the calibration curve data stored in the memory 33a is used. The gas concentration corresponding to the detection output after correction is obtained.

In addition, the control device disclosed in Japanese Patent Laid-Open No. 7-270315 is provided with a measurement sensor 20 that measures the concentration of the detected outgas for controlling the controlled device 50 in the atmosphere and outputs the measured concentration a. At the same time, a reference measurement sensor 25 for measuring the concentration of the same kind of gas in the atmosphere that does not contain the detected output gas and outputting the atmosphere base concentration b is provided, and the control means 10 subtracts the atmosphere base concentration b from the measurement concentration a. The reliability is improved by obtaining the control density c in which the secular change is corrected.
JP 2006-194776 A JP-A-7-270315

  However, in the aging correction method disclosed in Japanese Patent Application Laid-Open No. 2006-194976, the amount of decrease in sensitivity of the sensor is measured using a calibration gas, and a calibration calculation formula corresponding to the change is used, whereby the sensor Although the output is corrected, this method requires a standard gas for calibration during the calibration period, and also requires a structure that switches the flow path of the detected output gas. It has become. Therefore, the structure is complicated and it is difficult to reduce the size, and it is not suitable for a small-sized application such as a ventilation system that requires a continuous monitor.

  Further, in the aging correction method disclosed in Japanese Patent Laid-Open No. 7-270315, the output of a reference sensor placed in the atmosphere that does not contain the detected outgas and the measurement sensor placed in the detected outgas. By subtracting the output, output fluctuation due to aging of the sensor is corrected.

  However, an adsorption combustion type gas sensor that detects a VOC (volatile organic compound) gas or the like adsorbs and combusts a gas to be detected, and causes secular change due to debris or silicone poisoning attached to the catalyst surface. Therefore, in the same method as the aging correction method disclosed in Japanese Patent Laid-Open No. 7-270315, only the gas sensor placed in the atmosphere including the detected outgas is subject to aging due to burnout or silicone poisoning. Will occur and cannot be corrected.

  Therefore, in view of the above-described problems, the present invention provides a gas detection device that can satisfactorily correct a change in sensor sensitivity due to a secular change of a gas sensor when an adsorptive gas such as VOC is detected by a catalytic combustion type gas sensor. It aims to provide a gas detection method.

  In order to solve the above problems, the gas detector according to the first aspect of the present invention has a combustion temperature at which the detected gas burns in a state where the detected gas is adsorbed as shown in the basic configuration diagram of FIG. A catalytic combustion gas sensor 41 that measures the concentration of the detected gas when heated, and measures the air base concentration when heated to the combustion temperature in a state where the detected gas is not adsorbed; and the catalytic combustion Concentration output acquisition means for acquiring a measured concentration output representing the concentration of the detected gas measured by the gas sensor 41 and obtaining an air base concentration output representing the air base concentration measured by the catalytic combustion gas sensor 41 11a, storage means 12 for storing in advance the relationship between the amount of time and the air base concentration output, and the relationship between the amount of time and the sensor sensitivity, and acquisition of the concentration output When the difference between the measured concentration output Vg acquired at the stage 11a and the air base concentration output is equal to or less than a predetermined value, the air base concentration output acquired by the concentration output acquisition means 11a and the storage A time acquisition unit 11b that acquires a time elapse based on a relationship between a time elapse stored in advance in the unit 12 and the air base concentration output, the time elapse acquired by the time elapse acquisition unit 11b, Sensor sensitivity acquisition means 11c for obtaining sensor sensitivity based on the relationship between the amount of time and sensor sensitivity stored in advance in the storage means 12, the measured concentration output acquired by the concentration output acquisition means 11a, and the air base concentration Detection density acquisition means for correcting the sensor sensitivity acquired by the sensor sensitivity acquisition means 11c to obtain the detection concentration of the gas to be detected. Characterized in that it comprises a 1d, a.

  In order to solve the above-mentioned problem, the invention according to claim 2 is the gas detector according to claim 1, wherein the contact combustion type gas sensor 41 is energized to the first energization so that the gas to be detected is adsorbed. After being stopped for a stop period, energization is performed for a first energization period so as to reach the combustion temperature at which the adsorbed detected gas is combusted, and then the catalytic combustion type so that the detected gas is not adsorbed. After energizing the gas sensor 41 for a second energization stop period shorter than the first energization stop period, energization for a second energization period equal to the first energization period so that the combustion temperature is reached. And a concentration output acquisition unit 11a that acquires the measured concentration output during the first energization period and the air base concentration during the second energization period. Output Characterized in that the Tokusuru.

  In order to solve the above-mentioned problem, the invention according to claim 3 is the gas detector according to claim 2, wherein the concentration output acquisition means 11a acquires the air base concentration output as an integral value or an instantaneous value. It is characterized by.

  In order to solve the above-mentioned problem, the invention according to claim 4 is the gas detection device according to claim 3, wherein the concentration output acquisition unit 11a acquires the integral value or the instantaneous value a plurality of times and performs an averaging process. A value is acquired as the air base concentration output.

  The gas detection method according to claim 5, which has been made to solve the above-described problem, is a gas to be detected when the gas to be detected is heated to a combustion temperature at which the gas to be burned is combusted in a state where the gas to be detected is adsorbed to the catalytic combustion gas sensor. A measurement concentration output acquisition step for measuring the concentration of the gas to be detected and acquiring a measured concentration output representing the measured concentration of the detected gas; and when the detected gas is not adsorbed, the air is heated to the combustion temperature. An air base concentration output acquisition step for measuring a base concentration and acquiring an air base concentration output representing the measured air base concentration, and the measurement concentration output and the air base concentration output acquisition acquired in the measurement concentration acquisition step. When the difference in the air base concentration output acquired in the step is equal to or smaller than a predetermined value, the air base concentration output is stored in the storage means. A time amount acquisition step of acquiring a time amount based on the relationship between the time amount stored and the air base concentration output; the time amount acquired in the time amount acquisition step; The sensor sensitivity acquisition step for obtaining sensor sensitivity based on the relationship between the elapsed time and the sensor sensitivity, the measurement concentration output acquired in the measurement concentration acquisition step, and the air base concentration output acquisition step A detection concentration acquisition step of correcting the sensor sensitivity acquired in the sensor sensitivity acquisition step to acquire a detected concentration of the detected gas to a difference in air base concentration output, .

  According to the gas detection device of the first aspect of the present invention, it is possible to satisfactorily correct the secular change of the catalytic combustion type gas sensor with a minimum configuration such as a sensor and a detection circuit without requiring a standard gas for calibration. it can. It is small and easy to calibrate. Further, it is possible to continuously monitor the concentration of the gas to be detected.

  According to the gas detection device of the second aspect of the present invention, the atmosphere in which the gas to be detected exists by using the adsorption time difference method that changes the time length of the first energization stop period and the second energization stop period in the drive pulse signal. In particular, the air base concentration output can be detected in a state close to the zero gas state.

  According to the gas detection device of the third aspect of the present invention, it is possible to correct the secular change of the catalytic combustion type gas sensor using the integrated value or instantaneous value of the air base concentration output.

  According to the gas detection device of the fourth aspect of the present invention, it is possible to correct the secular change of the catalytic combustion type gas sensor using the integrated value or the average value of the instantaneous values of the plurality of air base concentration outputs.

  According to the gas detection method of the fifth aspect of the present invention, it is possible to satisfactorily correct the secular change of the catalytic combustion type gas sensor without requiring a calibration standard gas.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  As shown in FIG. 2, the gas detection device 1 according to the embodiment of the present invention includes a microcontroller unit (MCU) 10, a drive power supply 20, an output unit 30, and a gas sensor unit 40. A drive power supply 20, an output unit 30, and a gas sensor unit 40 are connected to the MCU 10.

  The MCU 10 stores a central processing unit (CPU) 11 that performs various processes and controls according to a predetermined program, a ROM 12 that is a read-only memory that stores programs for the CPU 11, and various data. At the same time, it has a RAM 13 which is a readable / writable memory having an area necessary for processing operations of the CPU 11 and an EEPROM 14 which is an electrically writable and erasable nonvolatile memory. The CPU 11 performs drive control of the gas sensor unit 40 by executing a drive control program or the like stored in the ROM 12.

  The CPU 11 includes a concentration output acquisition unit (11a in FIG. 1), an elapsed time acquisition unit (11b in FIG. 1), a sensor sensitivity acquisition unit (11c in FIG. 1), a detected concentration acquisition unit (11d in FIG. 1), and Acts as energization control means (11e in FIG. 1). The ROM 12 serves as storage means in the claims.

  As the drive power source 20, an existing commercial power source, a battery, or the like is used, and direct current power necessary for the operation of each unit such as the MCU 10 and the gas sensor unit 40 of the gas detection device 1 is generated and supplied to each unit for operation.

  The output unit 30 outputs various data such as the detected concentration and gas type of the gas to be detected in response to an instruction from the MCU 10. For example, an LCD (liquid crystal display) is used as the output unit 30 when displaying the detected concentration or the like, and an LED and a drive circuit are used when displaying the type of gas by color, blinking, or the like. Further, when outputting the gas detection result as an audible signal, a buzzer, a speaker, an output circuit, and the like are used.

  The gas sensor unit 40 includes a contact combustion gas sensor 41 that outputs a sensor output that is sensitive to the adsorbed gas to be detected, a drive unit 42 that drives the contact combustion gas sensor 41, and a sensor output unit 43.

  The catalytic combustion type gas sensor 41 includes a sensitive element part Rs having a catalyst capable of burning adsorbed gas on a platinum (Pt) heater and a compensating element part Rr having no catalyst, and both element parts are formed on the same chip. The drive pulse signal is supplied from the CPU 11, and the drive is intermittently performed so that the low temperature drive and the high temperature drive are alternated.

Specifically, as shown in FIG. 3, the sensitive element portion Rs includes a platinum (Pt) heater 412 and Pd / Al ₂ O made of alumina (Al ₂ O ₃ ) carrying a platinum group, for example, palladium (Pd). _The compensation element portion Rr includes the _three catalyst layers 413 and the platinum (Pt) heater 414 and the alumina (Al ₂ O ₃ ) layer 415.

Specifically, as shown in FIG. 3A, the catalytic combustion type gas sensor 41 includes a silicon oxide (SiO ₂ ) film, a silicon nitride (SiN) film, and a hafnium oxide (HfO) on a silicon (Si) wafer 411. ₂ ) An insulating thin film 418 made of a film or the like is formed, on which a platinum (Pt) heater 412 and a Pd / Al ₂ O ₃ catalyst layer 413 as the sensitive element portion Rs, and a platinum (Pt) heater as the compensating element portion Rr 414 and an alumina (Al ₂ O ₃ ) layer 415 are formed. Further, as shown in FIG. 3B, the concave portions 416 and 417 are formed by anisotropic etching to form thin film diaphragms Ds and Dr, respectively, thereby reducing the heat capacity.

  As described above, the sensitive element portion Rs and the compensating element portion Rr constitute the catalyst layer and the alumina layer on the platinum heaters 412 and 414, and the gas to be detected is adsorbed on the catalyst layer on the sensitive element portion Rs side. The alumina layer on the part Rr side has a structure in which the gas to be detected does not adsorb. When the heater temperature is heated to the combustion temperature of the detected gas, the detected gas adsorbed on the sensitive element portion Rs side burns, and the heater temperature on the sensitive element portion Rs side is higher than the compensating element portion Rr side. Become. Since the resistance values of the platinum heaters 412 and 414 increase constantly with respect to the temperature, the temperature difference due to the combustion described above can be detected as a difference in resistance values of the platinum heaters 412 and 414. The catalytic combustion type gas sensor 41 has a sufficiently small heat capacity and a high-speed response due to the above-described configuration.

  The drive unit 42 includes fixed resistors R1 and R2, a resistor Rb, and an npn transistor Q1 that form a bridge circuit together with the platinum (Pt) heater 412 of the sensitive element unit Rs and the platinum (Pt) heater 414 of the compensation element unit Rr. And have. In the bridge circuit, the collector of the transistor Q1 is connected to the connection point between the sensitive element portion Rs and the resistor R2, the base thereof is connected to the CPU 11 of the MCU 10 via the fixed resistor Rb, and the emitter thereof is grounded. A drive power supply 20 is connected to a connection point between the compensation element portion Rr and the resistor R1.

  In order to drive the drive unit 42 configured in this way, drive pulses with alternating low and high levels are generated by the MCU 10 and input to the base of the transistor Q1 via the fixed resistor Rb. When the low (L) level drive pulse is input to the base, the transistor Q1 is turned off, the energization of the bridge circuit is stopped, and when the high (H) level drive pulse is input. It becomes an ON state, and it will be in the energization state to a bridge circuit. Therefore, the power supply voltage is applied to and cut off from the bridge circuit in conjunction with the drive pulse.

  The sensor output unit 43 includes an amplifier 431, an A / D converter 432, and a variable resistor R3. The amplifier 431 is composed of an operational amplifier, and is inputted to its inverting input terminal and non-inverting input terminal, and is an intermediate voltage from the connection point of the resistors R1 and R2 and the connection point of the sensitive element part Rs and the compensation element part Rr in the bridge circuit. Amplify the potential difference between V- and V +. The output of the amplifier 431 is input to the A / D converter 432. The output of the A / D converter 432 is connected to the MCU 10 and outputs the result of A / D conversion to the MCU 10.

  Further, in the zero gas state (the state in which no gas to be detected exists), the bridge circuit is set to maintain an equilibrium state and no potential difference is generated between the intermediate voltages V− and V +. There is a slight variation in resistance, and Rr: Rs = R1: R2 may not be established even in a zero gas state (a state in which no gas to be detected exists). In such a case, when the bridge circuit becomes unbalanced, a potential difference is generated between the intermediate voltages V− and V +. This potential difference can be reduced to zero by finely adjusting the fixed resistors R1 and R2, but it is difficult to adjust minute variations, so that the variable resistor R3 connected to the drive power source 20 is used to adjust the amplifier 431. In this configuration, the offset voltage is inputted to the variable resistor R3 so that the potential difference can be adjusted to zero by varying the variable resistor R3.

  In the gas sensor unit 40 configured as described above, the DC voltage Vb is applied from the drive power supply 20 to the bridge circuit, and the intermediate voltages V− and V + at zero gas are both Vb / 2. Here, when the gas to be detected is adsorbed to the sensitive element portion Rs, a current flows through the heater by applying a high (H) level drive pulse signal to the transistor Q1, the sensor is heated, and the temperature of the sensor is heated. When the temperature rises to the combustion temperature of the detected gas (for example, 400 ° C.), the adsorbed detected gas burns. Thereby, the heater temperature on the sensitive element portion Rs side becomes higher than the compensation element portion Rr, and the intermediate potential V + becomes higher than Vb / 2. On the other hand, the intermediate potential V− of the fixed resistors R1 and R2 is always Vb / 2. Therefore, a potential difference is generated between the intermediate voltages V + and V−. However, as the concentration of the gas to be detected increases and more gas to be detected is adsorbed and burns, the potential difference increases.

  As described above, the gas adsorption amount to the sensitive element portion Rs changes with the change in the concentration of the gas to be detected, so that a change depending on the gas concentration can be obtained in the potential difference between the intermediate potentials V + and V−. This change is amplified as necessary by the amplifier 431, and the result of A / D conversion by the A / D converter 432 is output to the MCU 10 as a sensor output.

  In the above-described configuration, the catalytic combustion type gas sensor 41 having a sufficiently small heat capacity and capable of high-speed response is pulse-driven with drive pulse signals having two different periods so that the energization on period and the energization off period alternate. Base and gas sensitivity can be obtained. In particular, it is effective for adsorptive gases such as VOC (volatile organic compounds), and the gas concentration can be measured by making the adsorption time of the gas molecules to be detected on the sensor surface shorter than the saturated adsorption time. it can.

  FIG. 4 shows a timing chart of a waveform example of the drive pulse signal. In FIG. 4, the energization-off period a1 is a period sufficient for the catalytic combustion gas sensor 41 once heated to a high temperature by energization on in the period B0 to release heat and return to room temperature, and the detection gas is adsorbed. Is a period in which almost no progress. Normally, the energization off period a1 is 0.05 to 0.5 seconds, and differs slightly depending on the type of gas to be detected and the heat capacity of the sensor. This energization off period a1 corresponds to a second energization stop period in the claims.

  Thereafter, in the energization on period A1 (= B0) following the energization off period a1, the air base concentration output Va is acquired. The air base concentration output Va is a voltage signal representing the sensor output in a state where the gas molecules to be detected are not adsorbed on the sensor surface. The energization on period A1 corresponds to a second energization period in the claims. The energization off period b1 after acquiring the air base concentration output Va is a detected gas adsorption period, which is a period sufficient to adsorb until the detected gas molecules reach an amount that can be detected on the sensor surface. Usually, it is about 1 to 60 seconds. The energization off period b1 corresponds to a first energization stop period in the claims. Thereafter, in the energization-on period B1 (= B0) following the period b1, the measured concentration output Vg is acquired. This measured concentration output Vg is a voltage signal representing the sensor output in a state where the gas molecules to be detected are adsorbed and burnt on the sensor surface. The energization on period B1 corresponds to the first energization period in the claims. Then, the detected concentration output of the gas to be detected is obtained by the calculation of (measured concentration output Vg−air base concentration output Va). Thus, the period P including the air-based concentration measurement period Pa in the energization off period a1 and the energization on period A1 and the measured concentration measurement period Pb of the detected gas in the energization off period b1 and the energization on period B1 is defined as one cycle. The catalytic combustion type gas sensor 41 is driven by the energization pattern.

  5 to 7 are diagrams showing waveforms of the air base concentration output Va, the measured concentration output Vg, and the detected concentration output (Vg−Va) that are actually provided when the catalytic combustion type gas sensor 41 is driven as described above. It is. In addition, the vertical axis | shaft of FIG. 7 is expanded from FIG.

  Incidentally, as shown in FIG. 8, in the catalytic combustion type gas sensor 41, the sensitivity of the sensor output with respect to the gas to be detected decreases due to the secular change. This is because in the sensitive element portion Rs, burning residue accumulation and silicone poisoning occur on the catalyst surface. Due to these phenomena, the heat capacity of the sensitive element portion Rs increases, and a delay in temperature rise occurs in the transition period of the air base concentration measurement period Pa and the measured concentration measurement period Pb. Due to this delay, the detection value of the sensor output decreases, and by detecting the decreased value, the aging amount of the sensor can be acquired.

  In the present invention, a decrease in sensor sensitivity is corrected using the obtained amount of change with time. Hereinafter, correction of sensor sensitivity will be described.

  (1) First, as initial learning, the catalytic combustion gas sensor 41 at the beginning of aging is driven in a zero gas state (a state in which no gas to be detected exists), and the air base concentration measured in the energization on period A1 of the air base measurement period Pa. An output waveform Va is acquired. The acquired air base concentration output waveform Va in the energization on period A1 is integrated, and the Va integrated value is stored in the EEPROM 14 as an initial air base concentration value. The accuracy of the initial air base concentration value can be improved by repeatedly obtaining the Va integral value and performing an averaging process.

  (2) Next, normal detection gas detection is performed, and a measured concentration output waveform Vg measured in the energization-on period B1 of the measured concentration measurement period Pb is acquired. When the difference (Vg−Va) between the measured concentration output Vg and the air base concentration output Va is equal to or smaller than a predetermined value (that is, when the concentration of the gas to be detected is sufficiently low), the integration of the air base concentration output Va is performed. Get the value.

  (3) Next, based on the relationship between the elapsed time of the catalytic combustion type gas sensor 41 previously measured and stored in the ROM 12 and the Va integral value (see FIG. 9), the Va integral value acquired in the above (2) is obtained. Determine the corresponding amount of time. As shown in FIG. 9, the Va integral value is expressed as a numerical value that becomes smaller than the initial value with the passage of time (amount of time).

  (4) Next, the sensor sensitivity is obtained from the amount of time obtained in (3) above, using the relationship between the amount of time previously measured and stored in the ROM 12 and the sensor sensitivity (see FIG. 8). As shown in FIG. 8, the sensor sensitivity is expressed as a numerical value that becomes smaller than the initial value 1 with the passage of time (amount of time). The obtained sensor sensitivity is stored in the EEPROM 14 and then held until the condition (2) is satisfied.

  (5) Next, in order to eliminate the influence of fluctuations in the air base concentration, the air base concentration output Va is subtracted from the measured concentration output Vg (Vg−Va), and the subtraction result is corrected by the sensor sensitivity stored in the EEPROM 14. To obtain the detected density.

  (6) The above procedures (2) to (5) are repeated.

  In this way, it is possible to acquire the detected concentration that has been corrected for the secular change of the catalytic combustion type gas sensor 41. In the above description, the integrated value of Va in the energization on period A1 is used as the air base concentration output. However, an instantaneous value at a predetermined timing in the energization on period A1 is not limited to the integral value. Further, in (2) above, the operations of (3) and (4) are suspended until the Va integral value is acquired a plurality of times, and the accuracy is improved by using the averaged value of the plurality of Va integral values. be able to.

  Next, an example of the gas detection process executed by the CPU 11 in accordance with the gas concentration measurement process program stored in advance in the ROM 12 will be described with reference to the flowchart of FIG.

First, it is assumed that the standard aging characteristic (see FIG. 9) of the catalytic combustion type gas sensor 41 measured in advance and stored in the ROM 12 is as shown in the following regression equation.
itg_base = itg_base0 × exp ^(-0.018 × ^age) (1)
Here, itg_base: Va integral value at the time of detection of the gas to be detected, itg_base0: Va integral value at the beginning of secular change, age: Aging amount.

Further, it is assumed that the sensor sensitivity standard aging characteristic (see FIG. 8) of the catalytic combustion type gas sensor 41 measured in advance and stored in the ROM 12 is as shown in the following regression equation.
sense = exp ^(-0.0087 × ^age) (2)
Here, sense is sensor sensitivity.

  Therefore, first, it is determined whether or not the integrated value of the initial air base concentration output Va has been acquired (step S1). If the integrated value of the initial air base concentration output Va has not been acquired, the catalytic combustion gas sensor 41 in the early stage of aging is next driven by the drive pulse signal shown in FIG. The integrated value itg_base of the air base concentration output waveform Va measured in a state where no detection gas is present is acquired n times (step S2). Next, the Va integrated value acquired n times is averaged, and the itg_base average value is stored in the EEPROM 14 as the initial air base concentration value itg_base0 (step S3). Next, the initial value of the sensor sensitivity sense is stored in the EEPROM 14 (step S4).

  Subsequent to step S4 (or if the integrated value of the initial air base concentration output Va has been acquired in step S1), the initial air base concentration value itg_base0 and the initial sensor sensitivity sense stored in the EEPROM 14 are read ( Step S5). Next, the catalytic combustion type gas sensor 41 is driven by the drive pulse signal shown in FIG. 4, and the integrated value itg_base of the air base concentration output Va measured in the zero gas state (state in which no detected gas exists) in the air base measurement period Pa is obtained. Obtain (step S6). Next, a normal gas to be detected is measured in the measured concentration measurement period Pb, and an integrated value itg_gas of the measured measured concentration output Vg is acquired (step S7).

Next, as shown in the following equation (3), the sensor concentration sense is corrected by correcting the sensor sensitivity sense to a value obtained by subtracting the air base concentration output Va integrated value from the measured integrated value of the measured concentration output Vg to calculate the detected concentration gas. (Step S8).
gas = (itg_gas -itg_base) / sense (3)

  Next, it is determined whether or not the calculated detected concentration gas is lower than a predetermined constant value (step S9). If the detected concentration gas is not lower than the predetermined value, the process returns to step S6, and if it is not, the process proceeds to step S10.

In step S10, the integrated value itg_base of the air base concentration output Va and the initial air base concentration value itg_base0 are used to substitute the above equation (1) into the following equation (4) to calculate a time-lapse amount age.
age = ln (itg_base / itg_base0) / (− 0.018) (4)

  Next, the sensor sensitivity sense is obtained from the obtained age amount using equation (2) and stored in the EEPROM 14 (step S11), then the process returns to step S6, and the steps S6 to S11 are repeated thereafter.

  The sensor sensitivity sense acquired in step S11 is stored in the EEPROM 14 until the condition in step S9 (that is, the detected concentration gas is lower than a predetermined constant value) is updated. Retained.

  As is clear from the above description, step S6 in FIG. 10 corresponds to the air-based concentration output acquisition step in the claims, step S7 corresponds to the measured concentration output acquisition step in the claims, and step S8 corresponds to the detection in the claims. Corresponding to the density acquisition step, steps S9 and S10 correspond to the time-lapse amount acquisition step in the claims. Step S11 corresponds to the sensor sensitivity acquisition step in the claims.

  FIG. 11 is a diagram showing the relationship between the detected density output of a constant density before and after correction and the amount of time. From FIG. 11, it can be seen that the characteristic with the correction is almost constant over time as compared with the characteristic without the correction for the secular change of the catalytic combustion type gas sensor 41.

  As described above, the detected concentration of the gas to be detected is output after correcting the secular change of the catalytic combustion type gas sensor 41. In the above flow chart, when the concentration of the gas to be detected is sufficiently low in steps S9 to S11, the amount of time and sensor sensitivity are calculated and stored using itg_base. Since itg_base is low in sensitivity to the gas concentration, it can be handled as a zero gas state (a state in which no gas to be detected exists) in a low concentration environment. For example, when an accuracy of about 1 ppm is required, a detected gas concentration of about 0 to 0.3 ppm can be handled as a zero gas state. By handling in this way, detection accuracy can be improved.

  In the above flowchart, in steps S10 and S11, the amount of time age and the sensor sensitivity sense are estimated using the air base concentration detection value itg_base detected in step S6. For the integral value itg_base of Va and the integral value itg_base0 of the initial Va used when performing this estimation, it is possible to improve the detection accuracy by averaging a plurality of detection values.

As described above, according to the present invention, the following effects can be obtained.
(1) It is possible to correct the secular change of the contact combustion type gas sensor 41 with a minimum configuration such as a sensor and a detection circuit.
(2) No standard gas for calibration is required.
(3) Small and easy to calibrate.
(4) Continuous monitoring is possible.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to this, A various deformation | transformation and application are possible.

  For example, in the above-described embodiment, the adsorption time difference method of changing the time length of the energization off period a1 and the energization on period A1 in the drive pulse signal is used to acquire the air base concentration output Va and the measured concentration output Vg. . In this adsorption time difference method, by controlling the adsorption time to the catalytic combustion type gas sensor 41, the air base concentration output can be detected in a state close to the zero gas state even in the atmosphere in which the gas to be detected exists. Instead of this adsorption time difference method, the present invention absorbs the detected gas by passing the atmosphere in which the detected gas exists through a filter such as activated carbon to generate a zero gas state, and integrates the air base concentration output Va. Even if it is configured to acquire itg_base, it is possible to correct the aging degradation of the catalytic combustion type gas sensor 41 in the same manner. In the case of the filter method, it is possible to obtain a zero gas state in which almost no gas to be detected exists compared to the adsorption time difference method.

It is a basic composition figure showing the basic composition of the gas detector of the present invention. It is a block diagram which shows the structure of the gas detection apparatus which concerns on embodiment of this invention. The structural example of the contact combustion type gas sensor in FIG. 2 is shown, (A) is a top view, (B) is the XX sectional view taken on the line in (A). It is a timing chart of the example of a waveform of the drive pulse signal which drives a contact combustion type gas sensor. It is a figure which shows the air-base density | concentration output Va waveform which was actually carried out when the contact combustion type gas sensor was driven. It is a figure which shows the waveform of the measurement density | concentration output Vg and detection density | concentration output (Vg-Va) which were actually carried out when the contact combustion type gas sensor was driven. It is a figure which shows the waveform of the detection density | concentration output (Vg-Va) actualized when the contact combustion type gas sensor was driven. It is a characteristic view which shows the relationship between the amount of time and sensor sensitivity. It is a characteristic view which shows the relationship between the amount of time and the air base concentration output Va integral value. It is a flowchart which shows an example of the gas detection process which CPU in FIG. 2 performs. It is a figure which shows the relationship between the detection density output of the fixed density | concentration before correction | amendment, and after correction | amendment, and a time-dependent amount.

Explanation of symbols

1 Gas detector 11 CPU
11a Concentration output acquisition means (CPU)
11b Amount-of-time acquisition means (CPU)
11c Sensor sensitivity acquisition means (CPU)
11d Detected density acquisition means (CPU)
11e Energization control means (CPU)
12 Storage means (ROM)
14 EEPROM
41 Contact combustion gas sensor

Claims (5)

When the detected gas is heated to a combustion temperature at which the detected gas is burned in a state where the detected gas is adsorbed, the concentration of the detected gas is measured, and when the detected gas is not adsorbed, it is heated to the combustion temperature. A contact combustion gas sensor that measures the air base concentration when
Concentration output acquisition for acquiring a measured concentration output representing the concentration of the detected gas measured by the catalytic combustion type gas sensor and for acquiring an air base concentration output representing the air base concentration measured by the catalytic combustion type gas sensor Means,
Storage means for storing in advance the relationship between the amount of time and the air base concentration output, and the relationship between the amount of time and the sensor sensitivity;
When the difference between the measured concentration output acquired by the concentration output acquisition unit and the air base concentration output is equal to or less than a predetermined value, the air base concentration output acquired by the concentration output acquisition unit, An amount of time acquisition means for acquiring an amount of time based on the relationship between the amount of time stored in advance in the storage means and the air base concentration output;
Sensor sensitivity acquisition means for obtaining sensor sensitivity based on the amount of time acquired by the amount of time acquisition means and the relationship between the amount of time stored in advance in the storage means and sensor sensitivity;
The difference between the measured concentration output acquired by the concentration output acquisition means and the air base concentration output is corrected by the sensor sensitivity acquired by the sensor sensitivity acquisition means to acquire the detected concentration of the detected gas. Detection density acquisition means;
A gas detection device comprising: The gas detection device according to claim 1,
After energization of the catalytic combustion type gas sensor is stopped during the first energization stop period so that the detected gas is adsorbed, the first energization is performed so that the combustion temperature at which the adsorbed detected gas is burned is reached. Energized for a period of time, and then, after the energization of the catalytic combustion gas sensor is stopped for a second energization stop period shorter than the first energization stop period so that the detected gas is not adsorbed, the combustion Further comprising energization control means for controlling energization during a second energization period equal to the first energization period so as to be at a temperature;
The gas detector according to claim 1, wherein the concentration output acquisition means acquires the measured concentration output during the first energization period and acquires the air base concentration output during the second energization period. The gas detection device according to claim 2,
The gas detector according to claim 1, wherein the concentration output acquisition means acquires the air base concentration output as an integral value or an instantaneous value. The gas detector according to claim 3, wherein
The gas detector according to claim 1, wherein the concentration output acquisition unit acquires a value obtained by acquiring the integral value or the instantaneous value a plurality of times and averaging the acquired values as the air base concentration output. The concentration of the detected gas is measured when the detected gas is heated to the combustion temperature at which the detected gas burns in the state where the detected gas is adsorbed to the contact combustion type gas sensor, and the measured concentration output representing the measured concentration of the detected gas is output. A measurement concentration output acquisition step to be acquired;
An air base concentration output acquisition step of measuring an air base concentration when heated to the combustion temperature in a state where the detected gas is not adsorbed, and acquiring an air base concentration output representing the measured air base concentration;
When the difference between the measured concentration output acquired in the measured concentration acquisition step and the air base concentration output acquired in the air base concentration output acquisition step is equal to or less than a predetermined value, the air base concentration output and A time-lapse amount obtaining step for obtaining a time-lapse amount based on a relationship between a time-lapse amount stored in advance in a storage unit and the air base concentration output;
A sensor sensitivity acquisition step for obtaining a sensor sensitivity based on the time amount acquired in the time amount acquisition step and a relationship between the time amount stored in advance in the storage means and the sensor sensitivity;
The sensor sensitivity acquired in the sensor sensitivity acquisition step is corrected for the difference between the measured concentration output acquired in the measured concentration acquisition step and the air base concentration output acquired in the air base concentration output acquisition step. A detection concentration acquisition step of acquiring the detection concentration of the gas to be detected;
A gas detection method comprising:

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