JPH03140838A - Gas sensor - Google Patents

Abstract

PURPOSE:To measure the amount of gas with high resolving power on the basis of the frequency change of a surface elastic wave element by detecting the phase difference between the first and second synthetic signals as the change quantity of frequency of a surface elastic wave. CONSTITUTION:The resonance frequency of a surface clastic wave element 10 is changed by frequency DELTAf from reference resonance frequency fP by the adsorption of gas due to an adsorbing membrane 16 and a measuring signal changed by frequency (fP-DELTAf) is detected by a detector 10b to be outputted. The first reference signal (frequency fR) whose frequency is slightly different from the reference resonance frequency fP and the measuring signal are synthesized by the first synthesizing means 32 to fetch the first synthetic signal changed by the difference frequency (fP-fR-DELTA) of those frequencies. The second reference signal whose frequency is equal to the frequency fP and the first reference signal are synthesized by the second synthesizing means 34 to fetch the second synthetic signal changed by the difference frequency (fP-FR) thereof. Then, first and second synthetic signals is detected by a phase difference detection means 36. Next, the amount of the gas adsorbed on the adsorbing membrane can be measured on the basis of the phase difference with high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an improvement in a gas sensor that measures the amount of gas based on the frequency change of a surface acoustic wave element to which a gas adsorption film is attached.

Conventional technology LB (Langmuir-
A gas sensor that measures the amount of gas such as ethanol gas using an adsorption film having gas adsorption properties such as a J.D. Blodgell film has been considered.
A device that measures the amount of gas based on the fact that the oscillation frequency of the surface acoustic wave device changes in response to the change in mass caused by the gas adsorbed on the adsorption film attached to the surface acoustic wave device, such as a cut quartz plate. Proposed. For example, “”198
An example of this is the gas sensor described in 4p-G-1 of the 9th Japan Society of Applied Physics Conference Proceedings.

Problems to be Solved by the Invention However, conventional gas sensors of this type detect the oscillation frequency of the surface acoustic wave element itself and directly observe changes in that frequency, so it is not always possible to measure the gas amount with high resolution. There was a problem that it could not be measured.

The present invention has been made against the background of the above-mentioned circumstances, and its purpose is to enable gas amount to be measured with high resolution based on frequency changes of a surface acoustic wave element.

Means for Solving the Problems To achieve the object, the present invention provides an adsorption film having gas adsorption properties attached to a surface, and a surface acoustic wave generating surface acoustic wave at a predetermined reference resonance frequency. A gas sensor comprising a wave element and measuring the amount of gas based on a change in the frequency of the surface acoustic wave from the reference resonance frequency in response to a change in mass due to the gas adsorbed on the adsorption film, ) a detection means for detecting a surface acoustic wave of the surface acoustic wave element and outputting a measurement signal having the same frequency as the surface acoustic wave; and d) a first reference signal having a frequency slightly different from the reference resonance frequency. (C) a second reference signal whose frequency is equal to the reference resonance frequency; and the first reference signal, and outputs a second composite signal that changes at a difference frequency therebetween;
It is characterized by comprising a phase detection means for detecting a phase difference between the composite signal and the second composite signal as an amount of change in frequency of the surface acoustic wave.

Function and Effect of the Invention In such a gas sensor, the resonant frequency of the surface acoustic wave element is changed by the frequency Δf from the reference resonant frequency fp due to gas adsorption by the adsorption film, and the surface acoustic wave element is vibrated at the resonant frequency (rr−Δf). The surface acoustic wave is detected by the detection means, and its frequency Cry-Δf
) is output. Then, a first reference signal (
Frequency fR) and measurement signal are synthesized by the first synthesizing means, so that their difference frequency (rpf, -Δf
) is taken out, while a second reference signal whose frequency is equal to the reference resonant frequency f, and the first
By combining the reference signal with the second combining means, a second combined signal varying at the difference frequency (r, -r,) is extracted, and the first combined signal and the second combined signal are The phase difference between the two is detected by the phase detection means.

The phase difference between the first composite signal and the second composite signal corresponds to the phase change amount of the measurement signal, that is, the frequency change amount Δf, and based on this phase difference, the frequency change amount Δf is on the order of a decimal point or less. The amount of gas, such as the mass and gas concentration of the gas adsorbed on the adsorption membrane, can be measured with high accuracy.

In this way, in the present invention, the amount of frequency change Δ due to gas adsorption
Since f is determined from the phase change amount of the measurement signal, that is, the phase difference between the first composite signal and the second composite signal, on the order of a decimal point or less, the gas amount can be measured with high resolution. In this case, it is virtually impossible to obtain the amount of phase change of the measurement signal only from the measurement signal because the frequency of the measurement signal is high. By extracting the second composite signal, the phase difference thereof, that is, the amount of phase change of the measurement signal can be easily determined with high accuracy.

EXAMPLE Hereinafter, an example of the present invention will be described in detail based on the drawings.

In FIG. 1, reference numerals 10, 12, and 14 each indicate an ST-cut quartz plate surface acoustic wave device, and comb-shaped input electrodes 10a, 12a, and 14a and output electrodes 10b, 12b, and 14b are provided at both ends, respectively. There is. On the surface of the surface acoustic wave element 10, LBBi1 is formed by accumulating three monomolecular films of arachidic acid as an adsorption film having gas adsorption properties.
2 attached, and in that state the surface acoustic wave element 14
The surface acoustic wave element 12 generates a surface acoustic wave at the same resonant frequency f2 as the resonant frequency f2.
Surface acoustic waves are generated at a slightly different resonance frequency f++. Then, the surface acoustic wave element 1
0.14 input electrode 10a.

A high frequency voltage having the same frequency as the resonant frequency fp is applied from the crystal oscillator 18 to the output electrode 14a, and a surface acoustic wave having the resonant frequency f is generated, so that the output electrode 10b
, 14b to the resonant frequency f.

Electric signals SM and SR2 of the same frequency are output, respectively. Furthermore, a high frequency voltage having the same frequency as the resonant frequency fR is applied from the crystal oscillator 20 to the input electrode 12a of the surface acoustic wave element 12, and a surface acoustic wave having the resonant frequency fR is generated. An electrical signal SRI having the same frequency as the resonance frequencies f and l is output. The resonance frequency f is set to, for example, about 80 MHz, and the resonance frequency fR is set to about 79.9 MHz.

The surface acoustic wave element 10 is housed in a measurement container 24 into which ethanol gas 22 to be measured is introduced, and when the ethanol gas 22 is adsorbed to the LB film 16, the increase in mass is increased. The resonant frequency changes. Therefore, even if a high frequency voltage having the same frequency as the resonant frequency f is applied to the input electrode 10a, a surface acoustic wave having a resonant frequency (fr - Δf) that changes by Δf corresponding to the adsorbed mass of the ethanol gas 22 is generated. The frequency of the electrical signal SM output from the output electrode tab is also (
fp−Δf). In this embodiment, the resonance frequency f
1 corresponds to the reference resonance frequency, and the output electrode 10b corresponds to the detection means. Further, the electrical signals SM, SRI, and SR2 correspond to a measurement signal, a first reference signal, and a second reference signal, respectively.

The electric signals SM, SRI, and SR2 are each sent to an amplifier 2.
6.28.30, the electrical signals SM and SRI are supplied to the first combining means 32, and the electrical signals SRI and SR2 are supplied to the second combining means 34. These combining means 32 and 34 combine the respective supplied signals and output a combined signal that changes at the difference frequency, and the first combining means 32 outputs a combined signal with a difference frequency f0
The first composite signal SD varying by (-f P f *-Δf) is output, and the second composite signal 34 outputs the difference frequency f
A second composite signal SB varying at ++ (=fr fR) is output.

The composite signals SD and SB are each output from the phase detection means 3.
6, and their phase difference ΔΦ is detected. The phase detection means 36 is configured, for example, as shown in FIG. 2, and detects the composite signal SD.

SB is shaped into a rectangular wave by comparator circuits 38 and 40, respectively, and is supplied to counter circuits 42 and 44. The counter circuits 42.44 count the wave numbers of the composite signals SD and SB shaped into rectangular waves at predetermined intervals, for example, every 1 msec, and output the counted values to the latch circuits 46.48. is temporarily stored in latch circuits 46 and 48, and then subtracted by subtracter 50. Then, the subtracted value C1 is temporarily stored in the latch circuit 52.

The subtraction value C1 has one phase 2π of the phase difference ΔΦ as one unit.

Further, the rectangular composite signals SD and SB outputted from the comparator circuits 38 and 40 are combined with the pulse signal SP of frequency fc outputted from the crystal oscillator 54 to an AND circuit 5.
6. The first composite signal SD is supplied to the AND circuit 56 via the NOT circuit 58, and
The number of pulses C of the pulse signal SP" that has passed through the ND circuit 56
2 is counted by the counter circuit 60 at a predetermined timing and temporarily stored in the latch circuit 62.

FIG. 3 shows the first signal inverted by the NOT circuit 58.
5 is a time chart showing an example of a composite signal SD'', a second composite signal SB, and a pulse signal SP' that has passed through an AND circuit 56. Note that the first composite signal SD is inverted and
The composite signals SD and S are supplied to the D circuit 56.
When there is no phase difference ΔΦ between the LB film 16 and the LB film 16, the frequency of the electric signal SM is f.
In the case of r, the pulse number 02 becomes zero.

Here, the number of pulses 02 corresponds to a portion of the phase difference ΔΦ0 that is smaller than one phase 2π,
For example, if the frequency f is 100 MHz and the frequency difference between the resonance frequencies fr and f, that is, the frequency of the second composite signal SB is 100 kHz, then one phase 2π is 1/100.
It will be detected on the order of 0. That is, the phase detection means 36 detects the subtraction value CI and the number of pulses 02.
The phase difference ΔΦ between the composite signal SD and SB is 2π/10.
It is detected on the order of 00. Further, this phase difference ΔΦ corresponds to the amount of phase change of the surface acoustic wave of the surface acoustic wave element 10 due to gas adsorption, and means that the amount of frequency change Δf of the surface acoustic wave is determined on the order of 1/1000. do. Therefore, for example, if the frequency change Δf of the surface acoustic wave element 10 due to the adhesion of 50% concentration ethanol gas 22 to LBBi 12 is 150H2, the concentration of ethanol gas 22 will be measured with a resolution of about 3 ppm.

The subtraction value CI and the number of pulses 02 are read into the calculation means 66 via the data bus line 64, respectively. The calculation means 66 includes a CPLI6B, a RAM70.
It is equipped with a ROM 72, performs signal processing according to a program stored in the ROM 72 in advance while utilizing the temporary storage function of the RAM 70, and calculates the frequency change amount Δf on the order of 1/1000, for example, from the subtraction value C1 and the number of pulses Ct. In addition, based on a predetermined relational expression or data map between the gas adsorption amount and the frequency change amount Δf,
The mass of the ethanol gas 22 adsorbed by LBBi12 is determined, and the mass is converted into a % concentration and displayed on the display board 7.
Display on 4.

In this way, in the gas sensor of this embodiment, the amount of frequency change Δf due to gas adsorption is determined on the order of 1/1000, for example, based on the phase difference ΔΦ between the composite signals SD and SB. Concentration in ppm
It is possible to perform high resolution measurements on the order of .

Here, since the composite signals SD and SB are approximately 100 kHz, the phase difference ΔΦ can be detected on the order of 1 2 of 2π/1000 using the 100 MHz pulse signal SP. Δf can be determined on the order of 1/1000. That is, in order to directly detect the phase change of the 80 MHz electric signal SM on the order of 2π/1000, it is necessary to count 80 GHz pulse signals, which is virtually impossible.

Further, since the phase difference ΔΦ is measured in a time of about 1 msec, for example, it becomes possible to measure the gas concentration at high speed.

In addition, in this embodiment, since the electrical signals SRI and SR2 as reference signals are extracted using the surface acoustic wave elements 12 and 14, signal processing such as synthesis with the electrical signal SM, which is the measurement signal, is performed. In addition to being simple, since the surface acoustic wave elements 10, 12.14 are caused to resonate with high accuracy by the crystal oscillator 18.20, there is an advantage that high measurement accuracy can be obtained for the phase difference ΔΦ and also the gas concentration.

Note that after such a gas concentration measurement, a new gas concentration measurement can be performed by introducing N2 gas into the measurement container 24 to remove the ethanol gas 22 adsorbed on the LBBi12 and purify the LBBi12. can.

Although one embodiment of the present invention has been described above in detail based on the drawings, the present invention can also be implemented in other embodiments.

For example, in the above embodiment, the surface acoustic wave element 10°12.1
4, an ST cut crystal plate is used, but LiN
bO3 and Li2840. It is also possible to use surface acoustic wave elements such as .

Further, in the above embodiment, LBBi12, which is a stack of three layers of arachidic acid films, is used as the adsorption film, but the material, number of layers, etc. of this adsorption film may be changed as appropriate depending on the type of gas to be measured.

Further, in the embodiment, the electric signal SRI as a reference signal is
, SR2 are taken out from the surface acoustic wave element 1214, but it is also possible to use the electrical signals output from the crystal oscillators 18 and 20 as they are.

Furthermore, instead of the crystal oscillator 18.20, other 3 4 oscillators may be used to cause the surface acoustic wave elements 10, 12.14 to resonate.

Further, the phase detection means 36 of the above embodiment is configured to detect the phase difference ΔΦ on the order of, for example, 2π/1000, but this is set as appropriate depending on the resolution required by the frequency fc of the crystal oscillator 54. be done.

Although other examples are not provided, the present invention can be implemented with various modifications and improvements based on the knowledge of those skilled in the art.

[Brief explanation of the drawing]

FIG. 1 is a diagram illustrating the configuration of a gas sensor that is an embodiment of the present invention. FIG. 2 is a diagram illustrating the configuration of the phase detection means and calculation means in FIG. 1. FIG. 3 is a time chart illustrating the signals SD'SB and SP' in FIG. 2. 16: LB film (adsorption film) 22: Ethanol gas 32: First synthesis means 34: Second synthesis means 36: Phase detection means SM: Measurement signal SRI: First reference signal SR2:
2nd reference signal SD: 1st composite signal SB: 2nd composite signal

Claims (1)

[Scope of Claims] An adsorption film having gas adsorption properties is attached to the surface, and a surface acoustic wave element that generates a surface acoustic wave at a predetermined reference resonance frequency is provided, and the gas adsorbed on the adsorption film is A gas sensor that measures the amount of gas based on a change in the frequency of the surface acoustic wave from the reference resonance frequency in response to a change in mass due to A detection means that outputs a measurement signal of the same frequency as the surface acoustic wave, a first reference signal whose frequency is slightly different from the reference resonance frequency, and the measurement signal output from the detection means are combined, and the difference between them is detected. a first synthesizing means that outputs a first synthesized signal that varies with frequency; and a second synthesizer that synthesizes a second reference signal whose frequency is equal to the reference resonant frequency and the first reference signal, and that outputs a second synthesized signal that varies at a difference frequency between them. A gas sensor comprising: a second combining means for outputting a combined signal; and a phase detecting means for detecting a phase difference between the first combined signal and the second combined signal as an amount of change in frequency of the surface acoustic wave. .