JP3143060B2 - Gas detection method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas detection sensor for detecting and discriminating gas based on a change in resonance frequency using a quartz oscillator having an adsorption film on its surface, and a method for manufacturing the adsorption film. The present invention relates to a gas detection method including a method for correcting a variation in film thickness that occurs and a method for correcting a change in sensor response due to a temperature change.

[0002]

2. Description of the Related Art In a gas detection sensor, it is necessary to detect and discriminate a very small amount of combustion gas or organic compound gas generated by a fire or the like with high sensitivity. Conventionally, as a sensor for such an application, there is a sensor in which an adsorption film having a wide selectivity to a gas to be detected is applied to a quartz oscillator, and the resonance frequency is measured with time to detect the gas. Since the resonance frequency of the crystal oscillator decreases in proportion to the weight of the substance adsorbed on its surface, the gas adsorption amount can be obtained by measuring the resonance frequency.

[0003]

The gas detection sensor using a quartz oscillator having the above-mentioned adsorption film has the following problems. That is, a sensor using a crystal oscillator requires an expensive frequency counter to accurately measure the output resonance frequency. That is, when the frequency is measured using a low-cost multiplying and sampling circuit, a frequency dividing circuit, and a counter, the number of bits of the counter is limited, so that accurate measurement cannot always be performed. In addition, the thickness of the adsorption film used for this type of sensor tends to vary during the manufacturing process, and even if the same material and the same thickness are used, the characteristics of the sensor also tend to vary. This is a general trend. Furthermore, since the sensor output also fluctuates due to a temperature change, the output of the sensor may change due to the temperature change even though no gas is actually generated, which may cause a false detection. is there.

An object of the present invention can be realized by using a multiplying and sampling circuit, a frequency dividing circuit, and a counter which are relatively inexpensive, and is a highly accurate, small-sized gas detection apparatus having a film thickness and temperature correction function. It is to provide a sensor and a detection method.

[0005]

As means for solving the above-mentioned problems, as to measuring a frequency change using a multiplication & sampling circuit, a frequency dividing circuit, and a counter, the film thickness of the adsorption film before gas adsorption is measured. The change value of the frequency is calculated, not the frequency itself, using the change value of the counter output based on the change. That is, the present invention detects a gas to be detected by using a quartz oscillator sensor having an adsorption film on its surface and determining a change in the resonance frequency of the crystal oscillator due to adsorption of the gas to be detected onto the adsorption film. In the gas detection method, a change in resonance frequency is obtained from a difference between the resonance frequency of the reference oscillator and the resonance frequency of the sensor after gas adsorption, which is converted from the film thickness value before gas adsorption. It is.

Further, the variation in film thickness is corrected by multiplying the sensor output value by the film thickness ratio. Further, in order to eliminate the influence on the temperature change, two sensors having the same adsorption film are provided, and one of the sensors is covered with a cover made of metal or the like so as to cover the adsorption film of the sensor so as not to contact the gas. The effect of the temperature change is corrected by subtracting a constant multiple of the sensor output value from the sensor output value to be detected.

[0007]

FIG. 1 is a flow chart showing frequency change calculation, film thickness correction, and temperature correction showing an embodiment of a gas detection method according to the present invention. FIG. 2 is a block diagram showing a configuration of an embodiment of the gas detection method of the present invention. FIG.
In addition to the configuration of the temperature correction sensor in the above, the conventional technology can be applied mutatis mutandis. This will be described below with reference to FIGS. Multiplication & sampling circuit & frequency dividing circuit 8, counter 9
Is used to measure the resonance frequency fi of the crystal oscillator sensors 3 and 4 by dividing the frequency N and the frequency fr of the reference crystal oscillator.
(Constant) When the counter output is C, the difference fd between the resonance frequency of the sensor and the reference oscillator frequency is fd = fr−fi (1) fd = N · fr / C (2) Formula (1), Formula Fi = (1−N / C) · fr (3) is obtained from (2), and the resonance frequency fi is calculated using equation (3). This method is equivalent to the reciprocal method often used in frequency measurement.

To increase the resolution of the frequency measurement is to reduce the value of the frequency change with respect to the output change of the counter 9, and to increase the frequency division number N. However, since the number of bits of the counter is limited, if the value of N is too large, the counter 9 overflows and the true C
Is lost. In this kind of gas sensor, the change value of the resonance frequency of the sensor before sensing the gas and the resonance frequency after sensing the gas, that is, -Δfi,
It becomes the amount of gas adsorption. Here, when the amount of adsorption increases, fi decreases, and the sign of the change value of the resonance frequency becomes negative. Since what is required as the sensor output is not Δfi but −Δfi, equation (4) is derived from equation (1).

That is, .DELTA.fd =-. DELTA.fi (4), and .DELTA.fd can be obtained with high accuracy. With a counter having a limited number of bits, it is easier to accurately determine Δfd than to accurately determine fi.

Equation (5) is obtained by modifying equation (2).

C = (N · fr) / fd (5) When C and fd change by ΔC and Δfd by the adsorption film adsorbing the gas, respectively, C + ΔC = (N · fr) / (fd + Δfd) (6) When C is eliminated from the equations (5) and (6), -Δfi = Δfd = −fd [[1-1 / {1+ (fdΔC) / (N · fr)}] (7) . Here, fd is the difference between the resonance frequency of the sensor and the reference oscillator, and is obtained by converting the film thickness of the sensor into a frequency. The film thickness of the sensor is a known amount since it is measured after manufacturing.

Since it is possible that the film thickness may change due to aging or the like, it is necessary to obtain a more accurate change value of the resonance frequency in the equation (2) before measuring the gas adsorption amount. Is used to calculate fd (S003). In equation (7), the gas adsorption amount−Δ
It is not necessary to know the value of C to determine fi;
It suffices if the change ΔC in output is known. Even if the counter 9 overflows, ΔC can be obtained.
Since the range of C is considered to be equal to the number of bits of the counter 9, a wide range of frequency change can be measured. According to this method, the frequency change can be accurately calculated by using the low cost multiplication & sampling circuit & frequency dividing circuit 8 and the counter 9 (S004).

It is known that if the material of the sensor is the same, the amount of adsorbed gas is proportional to the film thickness of the sensor. When the gas is discriminated by the database created using the sensor having the film thickness fd _0, the sensor used naturally has the film thickness fd _0.
It must be d ₀ sensor. However, the film thickness generally varies during the manufacturing process of the sensor. The same material, thickness in order to correct the sensor fd in sensor characteristics of the film thickness of fd _0, the a sensor output of the thickness fd expression (7) fd ₀ / fd multiplies. That is, −Δfi · (fd ₀ / fd) = Δfd · (fd ₀ / fd) = − fd · (fd ₀ / fd) · [1-1 / {1+ (fd · ΔC) / (N · fr)} ] = − Fd ₀ · [1-1 / {1+ (fd · ΔC) / (N · fr)}] (8) Thereby, the obtained value becomes the same value as the sensor output of the film thickness of fd _0. . Thus, the sensor output due to the variation in the sensor film thickness generated in the manufacturing process can be corrected (S00).
5).

Further, in order to detect gas with high sensitivity, it is necessary to make a correction to eliminate the influence of the temperature change on the sensor output. Since the output of the sensor covered with the cover so that the adsorption film is not exposed to the gas corresponds to a temperature change, the sensor output s (t) at a certain time t
Subtracts a constant multiple of the output sc (t) of the sensor covered by the cover and sets s (t) −χ · sc (t) to eliminate the influence of the temperature change on the output. Here, χ is the output ratio of both sensors when the temperature is changed in the absence of gas. That is, if χ = s (t) / sc (t) (9), the value of χ differs depending on the sensor. As a result, the component corresponding to the temperature change included in the sensor output is subtracted, and only the gas adsorption amount can be extracted. In this way, the film thickness correction and the temperature corrected sensor output are used for gas detection,
It can be used for discrimination (S006).

In FIG. 2, a sensor cell 0 exposed to the atmosphere has a reference crystal oscillator 1, a crystal oscillator 3 provided with a gas adsorbing film 2 on its surface, and a gas adsorbing film 2 covered with a gas adsorbing film 2. There is provided a crystal oscillator 4 in which air is sealed so as not to be exposed to air and covered. The resonance frequency of these crystal oscillators 1, 3, and 4 is 9 MHz. Up to eight quartz oscillators 3 and 4 can be attached.
Has different adsorption characteristics by changing the material and the manufacturing conditions.

The crystal oscillator 3 is provided with an oscillating circuit 6, and the covered crystal oscillator 4 is provided with an oscillating circuit 5. The oscillating circuits 5 and 6 are provided with the corresponding crystal oscillator 3 and cover. The crystal oscillator 4 oscillates at the resonance frequency. The multiplexer 7 selects one of the oscillation circuits 5, and the multiplication & sampling & frequency division circuit 8
A signal having a frequency equal to the frequency difference between the outputs of the selected oscillation circuits 5 and 6 is output. The counter 9 measures the frequency of this signal and sends it to the computer 10. The computer 10 uses the output of the counter 9 and the film thickness of the gas adsorbing film 2 to calculate the gas adsorption amount represented by the frequency change, performs gas detection and discrimination, and displays the discrimination result on an indicator 11 such as an LED or CRT. To be displayed. When manufacturing multiple detectors,
Although it is desirable that each sensor is made of the same material and has the same characteristics, the film thickness of the sensor may vary. At this time, the sensor characteristics also vary. Therefore, the sensor output is multiplied by a constant with the film thickness ratio as a constant.

Next, the operation of the gas detector will be described. When each of the oscillation circuits 5 and 6 is put into the operating state, the reference crystal oscillator 1, the crystal oscillator 3, and the crystal oscillator 4 with the cover
Starts oscillating. Reference crystal oscillator 1, crystal oscillator 3,
Since the covered crystal oscillator 4 is in the same cell 0, the temperature of each sensor is the same. When gas is generated, adsorption of gas molecules to be detected starts on the gas adsorption film 2 on each crystal oscillator 3, and the resonance frequency of each crystal oscillator 3 shifts according to the amount of adsorption. Oscillation frequency shifts. This shift of the oscillation frequency is proportional to the mass of the gas to be detected adsorbed on the gas adsorption film 2 of each crystal oscillator 3.

On the other hand, since the generated gas is not adsorbed on the gas adsorption film 2 on the quartz oscillator 4 covered with the cover, the resonance frequency does not change. However, the crystal oscillator 4 with the cover
The amount of air adsorbed in the cover changes due to a change in temperature. That is, although the amount of air adsorbed on each crystal oscillator 3 changes due to a temperature change, the magnitude of the change is a constant multiple of the output of the crystal oscillator 4 covered with the cover. By subtracting a constant multiple of the output of the crystal oscillator 4 with the cover, the component corresponding to the temperature change from the output of each crystal oscillator 3 is canceled. In this way, it is possible to eliminate the cause of erroneous detection of the sensor output due to the temperature change.

[0019]

Embodiments of the present invention will be described below with reference to the drawings. A measurement result based on this example will be described. Sensor made of PTFE & PE (polytetrafluoroethylene and polyethylene) having a film thickness of fd = 50 kHz (corresponding to about 1.1 microns), N = 204
8, fr = 9 MHz, a 16-bit counter is used, and when the maximum value of C is 65536, an attempt is made to measure the frequency using a multiplication & sampling circuit, a frequency dividing circuit, and a counter without using the film thickness. Then, with no gas adsorbed, C = N · fr / fd shown in equation (5)
368640, and cannot be measured by this counter. Therefore, if the resolution is reduced and N = 256, C = 46080 and measurement becomes possible.
The resolution at 46080 is: fi (46080) -fi (46079) = fd (460
79)-fd (46080) = 1.09 (Hz). When the gas is adsorbed and C is reduced, the resolution is further reduced. For example, when the gas is adsorbed and Δfd = 10 kHz, the counter value becomes C = 38399, and the resolution at this time is fi (38399) −fi (38398) = fd (383).
98)-fd (38399) = 1.56 (Hz).

The amount of adsorption is calculated from equation (7) using the film thickness. For the film thickness fd = 50 kHz, a value measured after manufacturing the sensor is used, or N = 2 in the equation (2).
It is obtained from the counter value C = 46080 when 56 is set.
N = 2048 is used for measuring the resonance frequency change.
Normally, C decreases when gas is adsorbed, so that the sign of ΔC is negative. Since ΔC can be measured as many as the number of bits of the counter, −65536 <ΔC <0.
When the maximum ΔC = −65536, −Δfi (−6553)
6) = 10810.8 (Hz) is changed to the resolution −Δfi (−65536) − (− Δfi (−6553).
5)) = 0.2 (Hz). Further, when ΔC = 0, −Δfi (−1) − {− Δfi (0)} = 0.136 (H
z). That is, using this method, N = 2048 can be used in the same circuit, and about 10.8 kHz
Can be measured at a resolution of about 0.2 Hz or less. Of course, the larger the value of N, the higher the resolution.

FIG. 3 is a graph showing acetone 1000 when the film thickness of a sensor made of PTFE & PE was changed.
It is a plot of the amount of adsorption with respect to ppm. FIG.
A proportional relationship holds between the film thickness and the amount of adsorption, and the film thickness f
To correct a d (eg, 27.8 kHz = 0.6 micron) sensor (output 100 Hz) to the characteristics of a fd ₀ (55.6 kHz = 1.2 micron) film thickness sensor (output 200 Hz), a film Sensor output with thickness fd, ie -Δfi
Is multiplied by fd ₀ / fd, that is, by a factor of two, and it is known that equation (8) is effective as an equation for film thickness correction.

An embodiment in which film thickness correction is effective for discriminating a gas will be described. Figure 4 shows a PCTFE (polychlorotrifluoroethylene) sensor (0.82 micron film thickness) and PTFE &
Concentration 350 pp of PE sensor (film thickness 1.16 microns)
This is a plot of a vector obtained by normalizing the maximum adsorbed amounts a and b at 25 ° C. for m acetone (marked with □) and methanol at a concentration of 360 ppm (marked with ○) as shown in the following (Equation 1).

[0023]

(Equation 1)

Since these sensors have a wide selectivity of reacting not only with one kind of gas but with various kinds of gases, a gas based on a database of vector plots for each gas is used. Is determined.
The gas can be determined based on where this vector is plotted. Acetone and methanol can be discriminated based on whether the plots of both sensors for an unknown gas are close to □ or close to ○. Here, in particular, it is intended to selectively detect acetone.

<Embodiment 2> Here, another PTFE & PE
A sensor (0.47 micron film thickness) is manufactured and this sensor is used in place of the 1.16 micron film thickness PTFE & PE sensor. The determination of gas must refer to the plot of FIG. 4, but FIG. 4 cannot be used as it is because the thickness of the PTFE & PE sensor is different.
The vector for a concentration of 350 ppm acetone is x in FIG.
Although it looks like a mark, it clearly deviates from the plot of acetone and methanol in FIG. 4, and it is difficult to distinguish this gas as acetone.

The film thickness is calculated by using the film thickness correction equation of the equation (8).
Corrected and plotted output of the 16-micron PTFE & PE sensor is shown by the mark x in FIG. It is very close to the plot of acetone (marked with □), which indicates that this gas can be determined to be acetone. FIG. 7 is a chart showing the maximum adsorption amounts a and b of the used sensors.

FIG. 8 shows a sensor (thickness: 0.93 micron) (marked with ○) and a phenylalanine for temperature compensation with a cover when phenylalanine was used as a raw material when the temperature (marked with □) was changed in air. FIG. 9 is a characteristic diagram showing an output of a sensor (0.87 μm in film thickness) (marked by “X”) as a raw material. The appearance of sensor output in the absence of gas is due to temperature changes. Since the response component due to this temperature change is undesirable, it is canceled using the sensor output for temperature correction. It is clear that the sensor output for temperature correction has a higher correlation with the sensor output than the thermocouple output (temperature curve), and it can be expected that the temperature correction is more appropriate if a sensor for temperature correction is used. In this case, the coefficient χ is calculated from the sensor output s (t) to the sensor output sc (t) for temperature correction.
The result obtained by subtracting the value obtained by multiplying = 1 by s (t) −χ · sc (t) = s (t) −sc (t) (10) is plotted by a circle in FIG. Have been. s (t) is also plotted with □,
In comparison with this, the magnitude is almost 0, which indicates that the temperature correction is appropriate.

[0028]

As described above, the present invention uses a relatively inexpensive multiplying and sampling circuit, a frequency dividing circuit, and a counter to detect gas with high accuracy, small size, and film thickness and temperature correction functions. Sensors and detection methods can be provided. Further, since a change in sensor output due to a variation in film thickness caused in a manufacturing process is corrected, even if the film thickness is different, if the sensors are made of the same material, the adsorption characteristics can be standardized. Also, since the influence on the sensor output due to the temperature change can be eliminated, there is an excellent effect that erroneous detection due to the temperature change can be prevented.

[Brief description of the drawings]

FIG. 1 is a flowchart of frequency change calculation, film thickness correction, and temperature correction showing an embodiment of the gas detection method of the present invention.

FIG. 2 is a block diagram showing a configuration of an embodiment of the gas detection method of the present invention.

FIG. 3 is a characteristic diagram showing a relationship between a film thickness and a suction amount of a sensor using PTFE & PE.

FIG. 4 is a diagram showing the relationship between the normalized maximum adsorption amounts of acetone and methanol of the PCTFE sensor (0.82 μm in thickness) and the PTFE & PE sensor (1.16 μm in thickness).

5 is a diagram showing a PCTFE sensor shown in FIG.
FIG. 4 is a graph showing the relationship between the normalized maximum adsorption amount of acetone and methanol of the PTFE & PE sensor (film thickness of 0.47 μm) and the PTFE & PE sensor.

6 is a diagram showing a PCTFE sensor shown in FIG.
FIG. 7 is a diagram showing a superimposed relationship between normalized maximum adsorption amounts of acetone and acetone of a PTFE & PE sensor (0.47 μm in film thickness) with acetone in a PTFE & PE sensor.

FIG. 7 is a table showing the maximum amount of adsorption for each gas of each sensor used in FIGS. 4, 5 and 6;

FIG. 8 is an output characteristic diagram of a sensor (thickness: 0.93 μm) using phenylalanine as a raw material and a sensor (0.87 μm thick) using phenylalanine for temperature correction covered with a cover. .

FIG. 9 is an output characteristic diagram of an uncorrected sensor output and a temperature corrected sensor.

[Explanation of symbols]

0: Sensor cell 1: Reference crystal oscillator 2: Gas adsorption film 3, 4: Crystal oscillator for sensor 5, 6, Oscillator circuit 7: Multiplexer 8: Multiplication & sampling & frequency dividing circuit 9: Counter 10: Computer 11: Display vessel

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-5-312709 (JP, A) JP-A-4-369459 (JP, A) JP-A-7-260528 (JP, A) JP-A-6-260528 265459 (JP, A) JP-A-2-36350 (JP, A) Preprint of the 36th Automatic Control Alliance Lecture (October 1993) 407-408 (58) Field surveyed (Int. Cl. ⁷ , DB name) G01N 5/02 JICST file (JOIS)

Claims (3)

(57) [Claims] A gas for detecting a gas to be detected by using a sensor of a crystal oscillator having an adsorption film on a surface thereof and determining a change in a resonance frequency of the crystal oscillator due to adsorption of the gas to be detected to the adsorption film. In the detection method, a change in resonance frequency is obtained from a difference between the resonance frequency of the reference oscillator and the resonance frequency of the sensor after gas adsorption, which is converted from the film thickness value before gas adsorption. . 2. The gas detection method according to claim 1, wherein the output value of said sensor is multiplied by a film thickness ratio of said adsorption film to correct a change in a gas adsorption amount due to a variation in film thickness. 3. The method according to claim 1, wherein a change in the gas adsorption amount due to a temperature change is corrected by subtracting from the output value of the sensor a constant multiple of the output of the oscillator provided with the adsorption prevention cover for the gas to be detected. The gas detection method according to claim 1.