Method for extracting gas characteristics by acoustic measurement and apparatus for the same purpose

Abstract

To provide a method capable of extracting characteristics of an object gas by measuring physical characteristics specific to the object gas with a relatively simple device configuration, and device therefor.SOLUTION: A gas characteristics extraction method by acoustic measurement, comprises: using an acoustic resonator 100 including housings 120 and 130, acoustic wave output means 150 attached to one end of the housing 120, and sound pressure measurement means 140 disposed in the housing 130, to introduce an object gas into the acoustic resonator 100 while emitting an acoustic wave excited by a specific signal from the acoustic wave output means 150 into the acoustic resonator 100; causing the sound pressure measuring means 140 to measure sound pressure due to the acoustic wave in both a frequency domain and a time domain; and extracting characteristics of the object gas from acquired results.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method and apparatus for extracting gas characteristics by acoustic measurement, and more particularly to a method and apparatus for extracting gas characteristics by acoustic measurement in both the frequency domain and the time domain. [Background technology]

[0002] Measuring, sensing, and extracting gas properties is crucial for identifying chemical species. Gas detectors measure specific properties such as molecular weight, density, volatility, and vapor pressure. Various gas properties can be extracted using analytical models based on the measured signals, or by combining multiple sensors that measure different properties. Such high-precision measurements enable the identification of gas samples based on the properties of gas molecules, and are used in many fields, including agriculture, healthcare, medicine, safety, robotics, and environmental science.

[0003] Existing gas sensors come in various types, utilizing metal oxides, chemistristors, field-effect transistors, electrochemistry, surface acoustic waves (SAW), cantilevers, films, quartz oscillators, and microchannels. When the substance to be measured flows around the sensor element, the adsorption of the target molecules onto the sensor element changes the behavior of the sensor element, outputting a unique signal based on the characteristics of both the sensor element and the target gas. Because this type of sensor usually relies on the chemical affinity between the sensor element and the gas molecules, chemical selectivity can be specifically designed for certain gas species (types of target gases), but it remains difficult to determine physical parameters such as gas density (ρ) and vapor pressure, in addition to concentration (C), with a simple device. In contrast, gas sensors that measure physical properties do not depend on the chemical affinity between such sensor elements and target molecules, and can therefore be applied to all types of gases, making it possible to quantify physical parameters.

[0004] Non-patent documents 1 to 3 describe acoustic gas sensors using solid piezoelectric SAWs. However, these sensors use chemoreceptors to induce frequency shifts, so the measurement accuracy depends on the performance of the chemoreceptors, and also require effort for the synthesis and manufacture of the chemoreceptors. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] X. Qi, J. Liu, Y. Liang, J. Li, S. He, The response mechanism of surface acoustic wave gas sensors in real time, Japanese Journal of Applied Physics, 58(2019) 014001. [Non-Patent Document 2] L. Zhou, Z. Hu, P. Wang, N. Gao, B. Zhai, M. Ouyang, et al., Enhanced NO2 sensitivity of SnO2 SAW gas sensors by facet engineering, Sensors Actuators B: Chemical, 361(2022) 131735. [Non-Patent Document 3] N. Levit, D. Pestov, G. Tepper, High surface area polymer coatings for SAW-based chemical sensor applications, Sensors Actuators B: Chemical, 82(2002) 241-9. [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] The object of the present invention is to provide a method and apparatus for extracting the characteristics of a target gas by measuring its unique physical properties using a relatively simple apparatus configuration. [Means for solving the problem]

[0007] The features of the present invention for achieving the above objective are as follows.

[0008] [1] A method for extracting gas characteristics by acoustic measurement, comprising: using an acoustic resonator having a housing, an acoustic wave output means attached to one end of the housing, and a sound pressure measuring means disposed inside the housing, introducing a target gas into the acoustic resonator while emitting an acoustic wave excited by a specific signal from the acoustic wave output means into the acoustic resonator, measuring the sound pressure due to the acoustic wave in both the frequency domain and the time domain using the sound pressure measuring means, and extracting the characteristics of the target gas from the results obtained in the measurement step. Methods that include... [2] The method according to [1], wherein the measurement step includes measuring the signal output from the sound pressure measuring means using an acoustic wave excited with 1 / f noise having constant energy per octave to obtain a frequency response function of the sound pressure acting on the sound pressure measuring means, and the measurement in the time domain includes measuring the time variation of the signal output from the sound pressure measuring means using an acoustic wave excited with a sine wave of a fixed frequency. [3] The method according to [2], wherein the extraction step includes obtaining physical parameters of the target gas from the frequency response function obtained from the measurement in the frequency domain, and extracting feature quantities from the time variation of the output signal obtained from the measurement in the time domain. [4] The method according to [3], wherein the measurement in the frequency domain includes measuring the signal output from the sound pressure measuring means when the acoustic resonator is filled with the target gas to obtain a first frequency response function, and measuring the signal output from the sound pressure measuring means when the acoustic resonator is filled with purge gas to obtain a second frequency response function, wherein in the extraction step, the physical parameters of the target gas are obtained based on the selected resonance peak by comparing the first and second frequency response functions. [5] The method according to [3], wherein the measurement in the time domain includes introducing the target gas and the purge gas alternately into the acoustic resonator and measuring the time change of the signal output from the sound pressure measuring means, and in the extraction step, extracting feature quantities from the time change of the output signal. [6] The method according to any one of [3] to [5], further comprising applying principal component analysis or linear discriminant analysis to a data set obtained in the extraction step, which consists of physical parameters of the target gas from the measurement in the frequency domain and feature quantities from the measurement in the time domain. [7] The acoustic resonator is a single-ended tube with one end open and the other end closed, and is a (1 / 4 + 2n / 4) wavelength resonator (where n is a non-negative integer), or a double-ended tube with both ends open, and is a (1 / 2 + 2n / 2) wavelength resonator (where n is a non-negative integer), according to any one of [1] to [6]. [8] The method according to [7], wherein the acoustic resonator is the one-sided open tube and is a resonator of (1 / 4 + 2n / 4) wavelength (where n is an integer greater than or equal to 0). [9] The method according to [8], wherein the acoustic resonator is a quarter-wavelength resonator.

[10] An apparatus for extracting the characteristics of a gas by acoustic measurement, comprising an acoustic resonator having a housing, acoustic wave output means attached to one end of the housing, and sound pressure measurement means disposed within the housing, the acoustic resonator having an introduction part for introducing a target gas therein and a discharge part for discharging the internal gas to the outside, and configured to introduce the target gas into the acoustic resonator while emitting an acoustic wave excited by a specific signal from the acoustic wave output means into the acoustic resonator, and to measure the sound pressure caused by the acoustic wave by the sound pressure measurement means in both the frequency domain and the time domain.

Advantages of the Invention

[0009] According to the present invention, there are provided a method and an apparatus therefor that enable extraction of the characteristics of a target gas by measuring the physical characteristics specific to the target gas with a relatively simple apparatus configuration.

Brief Description of the Drawings

[0010] [Figure 1] A schematic cross-sectional view showing a specific configuration example of an acoustic resonator that can be used in the present invention. [Figure 2] A diagram showing the result of numerically simulating the resonance frequency fres of the second resonance of a 1 / 4 wavelength acoustic resonator fabricated in an example by finite element analysis. [Figure 3] A diagram showing the result of numerically simulating the frequency response function of the sound pressure acting on a microphone for various sound velocities c in an acoustic resonator fabricated in an example by finite element analysis. [Figure 4] A schematic diagram showing the configuration of a measurement system for gas measurement using an acoustic resonator fabricated in an example. [Figure 5A] A diagram showing the frequency response function obtained by introducing a target gas into a measurement chamber in the measurement in the frequency domain of an example. [Figure 5B] A diagram showing an enlarged view of the range (range including the resonance peak of the second resonance) surrounded by a dotted line in FIG. 5A. [Figure 5C]This figure shows a graph plotting the relationship between the resonant frequency fres and the sound velocity c (Y axis) against the gas density ρ (X axis) for the target gas. The coefficient of determination of the approximation curve was R²=1 for both the resonant frequency fres and the sound velocity c. [Figure 6A] The figure shows alternating cycles illustrating the frequency and time domain behavior of n-hexane, as well as the measurement results in the frequency and time domains of the example. The frequency response function inserted in the figure is the resonance peak of pure nitrogen and n-hexane in the frequency range of 5.5 to 8.5 kHz for Figure 5A. [Figure 6B] The figure shows the root mean square values ​​of the microphone output voltage in the time domain for different gases, relating to the measurement results in the frequency and time domains of the example. [Figure 6C] Regarding the measurement results in the frequency and time domains of the embodiment, Figure 6B shows the microphone output voltage in the third cycle, with the baseline value (ΔV) subtracted. [Figure 6D] The measurement results in the frequency and time domains of the example are shown in Figure 6C, which has been normalized. The response signal to water is shown in the inset. [Figure 6E] This figure illustrates the characteristic quantities of rise time, rise amplitude, fall time, and fall amplitude extracted from the time-domain measurement results, using the response signal to n-hexane shown in Figure 6C as an example, in relation to the measurement results in the frequency domain and time domain of the example. [Figure 6F] This figure shows the results of principal component analysis performed using the data sets extracted from the measurement results in the frequency and time domains of the example. [Figure 7A] The figure shows the frequency response functions obtained by introducing target gases with different n-hexane concentrations into the measurement chamber, relating to the results of concentration-dependent measurements using n-hexane in the examples. [Figure 7B]The figure shows the time-domain root mean square values ​​of the microphone output voltage for target gases with different n-hexane concentrations, relating to the results of concentration-dependent measurements using n-hexane in the examples. [Figure 7C] The figure plots the relationship between the amplitude level (ΔV) of the microphone output voltage and the n-hexane concentration in the target gas, as shown in the example, for concentration-dependent measurements using n-hexane. The straight line in the figure represents the regression line. [Figure 7D] This figure shows the results of principal component analysis performed using a data set extracted from measurement results in the frequency and time domains for target gases with different n-hexane concentrations, regarding the concentration-dependent measurements using n-hexane in the examples. [Modes for carrying out the invention]

[0011] The embodiments of the present invention will be described in detail below. The following description of the constituent elements may be based on typical embodiments of the present invention, but the present invention is not limited to such embodiments.

[0012] The present invention provides a method for extracting gas characteristics by acoustic measurement, which uses an acoustic resonator comprising a housing, an acoustic wave output means attached to one end of the housing, and a sound pressure measuring means disposed within the housing.

[0013] Figure 1 is a schematic cross-sectional view showing a specific example of the acoustic resonator described above.

[0014] The acoustic resonator 100 shown in Figure 1 has an upper housing 120 and a lower housing 130 connected by a substrate 110. The upper housing 120 and the lower housing 130 are electrically connected by an orifice 112 provided in the substrate 110. Furthermore, the upper housing 120 and the lower housing 130 are provided with sealing members (O-rings) 122 and 132, respectively, to ensure airtightness between the upper surface of the substrate 110 and the lower part of the upper housing 120, and between the lower surface of the substrate 110 and the upper part of the lower housing 130.

[0015] A sound pressure measuring means 140 is located inside the lower housing 130, and the sound pressure measuring means 140 is capable of receiving sound pressure from inside the upper housing 120 via the orifice 112 of the circuit board 110.

[0016] An acoustic wave output means 150 is attached to the upper end of the upper housing 120, that is, the end opposite to the side where the lower housing 130, on which the sound pressure measuring means 140 is located, is situated. The part of the acoustic wave output means 150 that emits acoustic waves is directed towards the inside of the upper housing 120. In addition, a sealing member (O-ring) 124 is provided between the upper end of the upper housing 120 and the acoustic wave output means 150 to ensure airtightness between the upper end of the upper housing 120 and the acoustic wave output means 150.

[0017] In addition, the upper housing 120 has openings 126 and 128 on both the left and right sides. One opening (for example, opening 126) functions as an introduction point for introducing the target gas into the upper housing 120 (i.e., the acoustic resonator 100), while the other opening (for example, opening 128) functions as an exhaust point for discharging the gas from inside the upper housing 120 (acoustic resonator 100) to the outside. In other words, in the acoustic resonator 100 shown in Figure 1, there is a chamber C consisting of the space inside the upper housing 120 and the orifice 112 of the substrate 110. Chamber C is configured to introduce the target gas into its interior through opening 126 and to discharge the gas from its interior to the outside through opening 128. Hereafter, such a chamber C will also be referred to as a measurement chamber. Note that the location and size of the openings 126 and 128 are not limited to the configuration shown in Figure 1 and can be designed as appropriate. Furthermore, while the measurement chamber C is narrowly defined as the range from the lower end of the acoustic wave output means 150 to the lower surface of the substrate 110 (the upper surface of the sound pressure measuring means 140) (indicated by x1 in Figure 1), it should be noted that in the context of the resonant wavelength of the acoustic resonator 100, which will be described later, the range of the measurement chamber C takes into account the position of the acoustic wave generating part 152 of the acoustic wave output means 150 (indicated by x2 in Figure 1) and the position of the sound pressure receiving part 142 of the sound pressure measuring means 140 (indicated by x3 in Figure 1) (i.e., indicated by X in Figure 1). In cases where the configuration (structure) of the components actually used as the acoustic wave output means 150 and the sound pressure measuring means 140 allows for the exclusion of one or both of the above values ​​of x2 and x3, it may be possible to use, for example, the sum of x1 and x2 as the effective range of the measurement chamber C. In addition, although Figure 1 depicts the substrate 110 as having a constant thickness for clarity, if the actual thickness of the material used as the substrate 110 is sufficiently small compared to the value of x1 (or the sum of x1 and x2), the effective range of the measurement chamber C may be the value obtained by dividing the sum of x1 and x2 by the thickness of the substrate 110 (i.e., the range from the top surface of the substrate 110 to the acoustic wave generating unit 152 of the acoustic wave output means 150).

[0018] In an acoustic resonator 100 having such a configuration, the acoustic wave output means 150 emits acoustic waves excited by a signal having a specific frequency into the acoustic resonator 100 (into the measurement chamber C), and the sound pressure measuring means 140 measures the sound pressure caused by these acoustic waves, thereby enabling the necessary acoustic measurement. Furthermore, in the above acoustic measurement, it is also possible to introduce gas into the measurement chamber C through the opening 126 while emitting acoustic waves from the acoustic wave output means 150 into the measurement chamber C, and measure the sound pressure caused by the acoustic waves at that time with the sound pressure measuring means 140.

[0019] In this invention, generally available micro-electromechanical systems (MEMS) microphones and speakers can be used as the sound pressure measuring means 140 and the acoustic wave output means 150, respectively. The substrate 110 may be an insulating substrate, a printed wiring board (PWB) with conductive wiring on or inside the insulating substrate, or a printed circuit board (PCB) with electronic components further mounted on it. If a PCB is used as the substrate 110, it is also possible to electrically connect the sound pressure measuring means (microphone) 140 and the acoustic wave output means (speaker) 150 to it and configure it to control their operation.

[0020] When selecting materials to constitute the housing (upper housing 120 and lower housing 130) of the acoustic resonator 100, it is preferable to select materials that are less likely to cause chemical interactions, such as adsorption and desorption of the gas introduced into the housing. This is because such chemical interactions may affect the extraction results of the gas properties by the method of the present invention. Specifically, fluororesins such as PTFE (polytetrafluoroethylene), PFA (perfluoroalkoxyalkane), FEP (perfluoroethylene propene copolymer), and ETFE (ethylene tetrafluoroethylene copolymer) can be suitably used as materials to constitute the housing of the acoustic resonator 100, particularly the upper housing 120 having the measurement chamber C.

[0021] In this invention, the characteristics of a target gas are extracted by using an acoustic resonator 100 having the above configuration and measuring the target gas in both the frequency and time domains.

[0022] In a typical embodiment of the present invention, it is preferable that the acoustic resonator 100 is configured to form a quarter-wavelength resonator. In the acoustic resonator 100 having the configuration shown in Figure 1, an acoustic resonator 100 that functions as a quarter-wavelength resonator can be configured by appropriately setting the value of length X, which is expressed as the sum of the distance x1 from the lower surface of the substrate 110 to the lower end of the acoustic wave output means 150 (the part that emits acoustic waves), the distance x2 from the lower end of the acoustic wave output means 150 to the acoustic wave generating part (e.g., the diaphragm of a speaker) 152 provided inside the acoustic wave output means 150, and the distance x3 from the lower surface of the substrate 110 to the sound pressure receiving part (e.g., the diaphragm of a microphone) 142 provided inside the sound pressure measuring means 140. Specific examples of such quarter-wavelength resonators will be described in the Examples section. Furthermore, if permitted by the design of the device, the acoustic resonator 100 can also be configured as a 3 / 4 wavelength resonator, a 5 / 4 wavelength resonator, or other resonator with a wavelength of (1 / 4 + 2n / 4) wavelength (where n is an integer of 1 or more). In addition, in the acoustic resonator 100 shown in Figure 1, a one-sided open tube (closed tube resonator) is formed with the upper end of the upper housing 120 being an open end and the bottom of the lower housing 130 being a closed end. However, if the bottom of the lower housing 130 is left open to form a two-sided open tube (open tube resonator), it may also be possible to configure the acoustic resonator 100 as a 1 / 2 wavelength resonator or a resonator with a wavelength of (1 / 2 + 2n / 2) wavelength (where n is an integer of 1 or more). That is, in one embodiment of the present invention, the acoustic resonator 100 may be a single-ended tube with one end being an open end and the other end being a closed end, and may be a resonator with a wavelength of (1 / 4 + 2n / 4) wavelengths (where n is an integer greater than or equal to 0), and in another embodiment, the acoustic resonator 100 may be a double-ended tube with both ends being open ends, and may be a resonator with a wavelength of (1 / 2 + 2n / 2) wavelengths (where n is an integer greater than or equal to 0).

[0023] By using an acoustic resonator 100 configured to form a (1 / 4 + 2n / 4) wavelength resonator or a (1 / 2 + 2n / 2) wavelength resonator (where n is an integer greater than or equal to 0), more specifically a 1 / 4 wavelength resonator, it becomes possible to measure the sound pressure significantly increased at the resonant frequency using the sound pressure measuring means 140. This resonant frequency is given by the sound velocity c and density ρ of the gas inside the acoustic resonator 100 (inside the measurement chamber C). That is, when the target gas is introduced into the acoustic resonator 100 (inside the measurement chamber C) from the opening 126, the density ρ and sound velocity c of the gas inside the measurement chamber C change, and the sound pressure wave emitted from the acoustic wave output means 150 changes. The change in the resonant frequency of the gas inside the measurement chamber C, caused by this change in sound pressure wave, can be experimentally obtained by measurement in the frequency domain, specifically by a standard 1 / f equioctave pink noise test in the audible frequency range. Furthermore, time-series signals obtained from measurements in the time domain under constant (fixed) frequency conditions exhibit unique profiles depending on the type of target gas and the concentration of specific gases contained within it. By extracting multiple features from these profiles, multidimensional data analysis becomes possible. Based on this multidimensional data (data set) obtained from both frequency- and time domain measurements, it becomes possible to clarify the type of target gas and the concentration of specific gases contained within it.

[0024] Compared to existing acoustic gas sensors using solid piezoelectric SAWs as described in Non-Patent Documents 1-3, the device of the present invention does not use chemoreceptors or the like to induce the frequency shift mentioned above, is specific to the characteristics of the target gas, and has an audible resonant frequency (f res It is highly useful in measuring the speed of sound c of various gases. The speed of sound c of various gases can be measured using photoacoustic resonators and Helmholtz resonators, but these are usually used to monitor specific pure gases with well-known properties.

[0025] In contrast, the present invention, as specifically described in the examples below, performs measurements on a gas prepared using headspace gas from a sealed vial containing a specific chemical substance, and from the results, the sound velocity c and density ρ can be determined as physical parameters specific to the type of target gas. Furthermore, by measuring the time-series change in sound pressure in the time domain when the target gas is introduced into the measurement chamber, multidimensional data specific to the type of target gas can be obtained, and the chemical properties of the target gas can be obtained from this multidimensional data.

[0026] In addition, the present invention also makes it possible to extract the characteristics of a target gas by using physical parameters (such as sound velocity and density) obtained from measurements in the frequency domain and multidimensional data obtained from measurements in the time domain, and by performing various statistical processing and related machine learning that express the degree of similarity between data sets in some form based on the characteristics of the data sets, such as Principal Component Analysis (PCA) and Linear Discriminant Analysis (LDA).

[0027] The present invention will be described in more detail below with reference to examples. Please note that the following examples are not intended to limit the present invention, but rather to aid in its understanding. [Examples]

[0028] [Device configuration] In this embodiment, an apparatus (100) having the same configuration as described with reference to Figure 1 was fabricated. A commercially available MEMS microphone (ADMP401-sparkfun) was used as the sound pressure measurement means (140), and one earbud of a commercially available earphone (ZNS-0111-BEIY, manufactured by Boesklenn) was used as the acoustic wave output means (150). A PCB was used as the circuit board (110), with a PTFE upper enclosure (120) placed on one side of the PCB and a 3D-printed lower enclosure (130) placed on the other side of the PCB.

[0029] In the fabricated device, the length of the space inside the upper housing (measurement chamber C) into which the target gas is introduced (the value of symbol X shown in Figure 1, i.e., the distance from the diaphragm inside the speaker to the diaphragm inside the microphone) was approximately 17 mm. Dividing this length by the distance from the bottom surface of the PCB to the microphone diaphragm and the thickness of the PCB, the result was approximately 14 mm (corresponding to the distance from the top surface of the PCB to the speaker diaphragm). The diameter of the measurement chamber (inner diameter of the upper housing) was 6 mm, which is the same size as the inner diameter of the O-ring (124) fitted around the speaker to ensure airtightness between the speaker and the upper end of the upper housing. The airtightness of the two components was maintained by using a clip to hold the speaker and the O-ring in place.

[0030] With the above configuration, a quarter-wavelength acoustic resonator is formed on the upper end of the upper housing, having an introduction section (126) that allows the target gas to be introduced into the acoustic resonator (into the measurement chamber) and an exhaust section (128) that allows the gas inside the measurement chamber to be discharged to the outside. Here, the resonant frequency in the quarter-wavelength acoustic resonator is given by the following equation.

number

[0031] The results are shown in Fig. 2. The f of the nitrogen gas calculated by the FEA shown in Fig. 2 res was 18.737 kHz (c = 349 [m / s]). In Fig. 2, the line indicated by the symbol D-D’ shows the position of the diaphragm inside the microphone, arranged to contact the part of the substrate bottom surface where the orifice is provided.

[0032] In this embodiment, in order to more accurately obtain the sound velocity c of the target gas, the second resonance was utilized for the following three reasons. (1) Clear resonance peak: Around the frequency range of the first resonance (fundamental vibration), the resonance of the speaker diaphragm and the Helmholtz resonance between the orifice and the lower housing volume overlap, but there are no other large resonances around the second resonance, and the resonance peak is clear. (2) Large frequency change amount (Δf res ): For the second resonance, the frequency change due to the change in gas characteristics is three times that of the first resonance, making it easier to distinguish the sound velocity c between different gases. (3) Being in the audible range: The f of the second resonance res is 20 kHz or less, and simple audio equipment (for example, the minimum sampling rate of DVD audio quality is 44.1 kHz) can be used. When the resonator is filled with different gases, f res changes due to the changes in density ρ and sound velocity c. The sound velocity c of an ideal gas is given by the following equation.

Equation

Equation

[0033] [Experimental Procedure] Figure 4 is a schematic diagram showing the configuration of a measurement system for gas measurement using the device described above. In the measurement system shown in Figure 4, the first mass flow controller (MFC1) is connected to a nitrogen line and transports the headspace gas (sample gas), which is the vapor of the liquid sample contained in the vial, to the mixing chamber. The second mass flow controller (MFC2) introduces pure nitrogen into the mixing chamber, thereby diluting the sample gas transported via MFC1 to a predetermined ratio. The mixed gas (target gas) thus prepared is then introduced into the measurement chamber (quarter-wavelength acoustic resonator). The vial used was made of glass with a volume of 20 mL and contained 3 mL of liquid sample.

[0034] Using this measurement system, two different acoustic wave-based measurements were performed in both the frequency and time domains to extract various information about the properties of the target gas, obtaining the density ρ, sound velocity c, and other multidimensional data of the target gas.

[0035] In frequency domain measurements, to obtain FRF, the acoustic wave was excited with 1 / f isooctave pink noise with an amplitude of 10 mV. On the other hand, in time-domain measurements, acoustic waves are V in Excitation was performed with a fixed-frequency sine wave of the form ×sin(2πft). Here, V in θ is the input voltage to the speaker (mV), f is the test frequency (Hz), and t is the time (s).

[0036] A PXI5406 (NI) was used for voltage input to the speaker, and a PXI5922 (NI) was used for measuring the microphone output. The bias voltage to the microphone was set to 3V for all measurements. The typical laboratory temperature during measurements was 20°C. All measurements were performed under normal laboratory conditions to evaluate the feasibility of data acquisition under a constant noise environment.

[0037] In the frequency domain measurements (pink noise tests), measurements were performed under the condition of acquiring 200 kilosamples (kS) at a sampling rate of 200 kHz (i.e., data was acquired at 1 Hz). In addition, to reduce fluctuations due to ambient noise sources, the results of 100 pink noise tests were averaged. For time-domain time-series measurements, the measurements were performed under conditions that allow for 10Hz measurement using a standard microphone input terminal, i.e., acquiring data at a sampling rate of 48kHz for 4.8kS. Furthermore, in time-domain measurements, a ±500Hz bandpass filter was used to eliminate the influence of other unwanted noise sources. For example, at a test frequency f = 6.9 kHz, a 6.4–7.4 kHz bandpass filter was applied. After collecting the filtered data (4.8 kHz), the root mean square (RMS) value of the collected time-series data was calculated for the change in microphone output voltage (ΔV) at each sampling interval (100 ms).

[0038] [Experimental Results] The following details these measurement results.

[0039] [Measurement in the frequency domain: Frequency response function of target gas prepared from various liquid samples] The following liquid samples were used: water (ultrapure water, produced using a Merck Millipore ultrapure water production system), methanol (Kanto Chemical, purity >99.8%), ethanol (Fujifilm Wako Pure Chemical Industries, purity >99.5%), ethyl acetate (Kanto Chemical, purity ≥99.5%), toluene (Wako Pure Chemical Industries, purity ≥99.5%), acetone (Sigma-Aldrich, purity ≥99.5%), and n-hexane (Fujifilm Wako Pure Chemical Industries, purity ≥96.0%).

[0040] The vapor of the above liquid sample (headspace gas from the vial) was transported to the mixing chamber by MFC1 set to 2 sccm, diluted with pure nitrogen supplied from MFC2 set to 8 sccm, and a mixed gas (target gas) containing the sample gas at a concentration of 20% and a flow rate of 10 sccm was prepared and introduced into the measurement chamber of the acoustic resonator.

[0041] In the frequency domain measurements, the target gas was introduced into the measurement chamber for 4 minutes, and then the FRF was determined using pink noise. The pink noise test was performed 100 times, with a time difference of 0.8 seconds between each test, and the FRF was averaged over approximately 3 minutes. During this time, the target gas was continuously introduced into the measurement chamber.

[0042] As shown in Figure 5A, each target gas exhibited its own unique resonance peak, and it was confirmed that the frequency and intensity of the resonance peak differed depending on the target gas. In all target gases, the maximum resonance peak was observed around 6.9–7.0 kHz, which is composed of at least the following multiple resonances: • Basic quarter-wavelength resonance (estimated at 5.8 kHz by FEA), • The speaker's primary resonance (experimentally measured in an open environment and confirmed to be 6.6 kHz), and • The fundamental Helmholtz resonance between the PCB orifice and the lower enclosure housing the microphone (estimated at 9kHz by FEA).

[0043] Apart from the convolution of multiple resonance peaks in the 5-9 kHz range, there is a distinct resonance peak in the 17-19 kHz range, which corresponds to the second-order resonance (n=3, 3rd harmonic) within the measurement chamber for different gases. As described in the "Apparatus Configuration" section above, the second-order resonance was selected as the primary resonance for determining the sound velocity c of each target gas, and an enlarged view of the area around this resonance peak is shown in Figure 5B. The nine graphs in Figures 5A and 5B, from left to right with respect to the peak position in Figure 5B, represent n-hexane, acetone, ethyl acetate, toluene, air, ethanol, methanol, water, and pure nitrogen (N2).

[0044] The sharp peaks observed in the FRF shown in Figures 5A and 5B are observed occasionally regardless of the gas flow rate, suggesting they are due to acoustic noise in the laboratory environment. As shown in Figure 5B, the clear and accurate observation of the second-order resonance peak indicates the possibility of determining the sound velocity c for different gases even in noisy environments. Note that "Air" refers to the state where MFC1 and MFC2 are set to 0 sccm and the gas flow path is open. Under these conditions, the well-known values ​​of air density ρ and sound velocity c can be used as reference values.

[0045] f of each target gas res This was determined by Lorentz fitting. Assuming the speed of sound c for nitrogen is 349 m / s (at 20°C), the correction length L = L was obtained from the design value of the measurement chamber length of approximately 14 mm using the above equation (1). eff The value was calculated to be 13.92 mm. A manometer (1500 N, Hodaka Seisakusho) was connected to the gas flow path, and the pressure of each target gas was checked, showing the same value (0.5 kPa) for all target gases. This means that the pressure of each target gas is equivalent to 101.8 kPa, which is the sum of atmospheric pressure (101.3 kPa) and the flow pressure caused by both MFCs (0.5 kPa). Considering that the pressure in the measurement chamber is constant, the equation for an ideal gas can be rearranged as follows for the gas density ρ.

number

[0046] The resonance frequency f of all target gases used in this embodiment, including the measurements described later. res Table 1 shows the results, including density ρ, sound velocity c, and average molar mass Mw. The order of the target gases shown in Table 1 is based on their resonant frequencies f. res This is in ascending order. The resonant frequency f of the target gas compared to pure nitrogen (N2) res The reason for the decrease is that, according to the relationship between equations (1) and (3) above, the sound velocity c of each target gas decreases, and consequently, the gas density ρ increases via a power law (Figure 5C) (Figure 5B includes an arrow line and the label "Increasing ρ"). The relationship expected from equation (3) is ρ -0.5 Therefore, f is relative to the actual measurement results. res Both c and y show an exponent of -0.5, indicating good agreement.

[0047] [Table 1]

[0048] [Time-domain measurement: Time-domain response of target gases prepared from various liquid samples] The advantage of the acoustic resonator used in this invention is that it can operate in both the time domain and the frequency domain, thereby obtaining a unique multidimensional dataset for identifying the target gas. Based on this dataset and several computationally obtained parameters (e.g., Mw and ρ shown in Table 1), it becomes possible to identify the type of target gas. The acoustic wave equations in both the space and time domains are given by the following equations.

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[0049] For time-domain measurements, the mixed gas (target gas) prepared as described above was introduced into the measurement chamber of the acoustic resonator, and sampling and purging were performed alternately. During sampling, the flow rates of MFC1 and MFC2 were set to 2 sccm and 8 sccm, respectively, and during purging (pure nitrogen), the flow rates of MFC1 and MFC2 were set to 0 sccm and 10 sccm, respectively. Sampling and purging were performed for 4 minutes, and measurements were taken for each target gas in three consecutive cycles to achieve a near-steady state. The sample gas concentration in the target gas was varied by adjusting the flow rate ratio of MFC1 and MFC2 while maintaining a total flow rate of 10 sccm.

[0050] For time-domain measurements, a constant test frequency f = 6.9 [kHz] was used. This is because the resonance peak shifts to the lower frequency side depending on the characteristics of the target gas (inset in Figure 6A), and f = 6.9 [kHz] is the maximum f for nitrogen. res Less than =7.46[kHz] and the maximum f of n-hexane. res The frequency range was selected to cover approximately 6.952 kHz. Furthermore, at this test frequency, the power input to the speaker was kept to a minimum (V in all time-domain measurements). in It can be measured at 30 [mV]. While frequency domain measurements yield a single physical parameter, time domain measurements allow for the extraction of multidimensional datasets.

[0051] Figure 6A shows alternating cycles illustrating the frequency and time domain behavior of n-hexane. At f = 6.9 [kHz], the magnitude of the FRF changes when the acoustic resonator is filled with n-hexane (sampling step: Gas Sampling) and when it is filled with nitrogen (purge step: N2Purging). When gas sampling is performed, the amplitude of the corresponding FRF increases in the frequency domain, and the microphone output voltage increases in the time domain (with n-hexane, some clipping occurs when the microphone reaches its maximum voltage). This process is reversibly measured as an increase and decrease in output voltage during the sampling and purge steps, respectively.

[0052] Figure 6B shows the RMS of the microphone output voltage in the time domain for different target gases. This indicates that sampling and purging cause sample gas molecules in the target gas to enter and exit the acoustic resonator, and that this is reversible for all target gases. The seven graphs in Figure 6B, in descending order of the change in output voltage in each cycle, are n-hexane, acetone, ethyl acetate, toluene, ethanol, methanol, and water. The slight difference between the first cycle and the last two cycles is due to the change in the headspace gas in the vial from a static state to a dynamic state due to the nitrogen carrier gas in the first cycle. The difference in output voltage from the lowest (water) to the highest (n-hexane) ranges from -3 to 500 mV when the baseline value (ΔV) of the third cycle is subtracted, as shown in Figure 6C. The correspondence between the seven graphs in Figure 6C and the types of target gases is the same as in Figure 6B described above. It has been clearly shown that the signal intensity and shape of rise and fall times differ in the time domain for each type of target gas, and multidimensional data can be extracted from these differences.

[0053] To further characterize these differences, the normalized responses are plotted in Figure 6D. While most of the target gases exhibited conventional first-order responses with varying time constants, water vapor showed a very different signal, increasing for the first 20 seconds, then decreasing, resulting in a lower RMS output voltage compared to the reference nitrogen. Also, n-hexane (high Mw and high vapor pressure) showed the fastest rise, while toluene (high Mw and low vapor pressure) showed the slowest rise. This is likely due to toluene having a high Mw and low vapor pressure compared to acetone (low Mw and high vapor pressure) and ethanol (low Mw and low vapor pressure).

[0054] Here, we will explain the features extracted from the time-domain measurement results (Figure 6C): rise time, rise amplitude, fall time, and fall amplitude. The rise amplitude and rise time are defined as the amplitude level (ΔV) between 10% and 90% during a period of 1200 to 1440 seconds, and the time it takes to reach this value, respectively. Similarly, the features of fall amplitude and fall time are defined for a purge cycle of 1440 to 1680 seconds, in the same 10% to 90% intervals, for the decrease in amplitude (ΔV) and the time required to achieve this decrease.

[0055] Figure 6E outlines these terms, using the response signal for n-hexane shown in Figure 6C as an example. In addition, Figure 6F shows the results of principal component analysis (PCA) performed using the main information extracted from the signals in both domains. This PCA confirmed that each sample exhibited different behavior, and that these could be distinguished by using either the frequency domain or the time domain. For example, in the frequency domain, ethanol and methanol have a weight on the speed of sound c compared to other chemicals, but it is difficult to distinguish alcohols by these specific features. Also, rise and fall times are weighted for ethyl acetate and toluene, but the amplitude of the rise and fall is most related to n-hexane and acetone. This is because there are differences in vapor pressure, gas density ρ, and average molar mass Mw among these four samples. In any case, by combining the multidimensional data extracted from the measurement results in the time domain with the speed of sound c obtained from the measurement in the frequency domain, a multidimensional dataset capable of distinguishing different target gases can be obtained.

[0056] [Concentration dependent measurement] This section describes measurement results in both the frequency and time domains using six different target gases with varying n-hexane concentrations.

[0057] The n-hexane concentrations in the target gases used were 6%, 8%, 10%, 13%, 16%, and 20%. The method for preparing the 20% n-hexane mixed gas was as described above. Mixed gases of other concentrations were prepared by adjusting the flow rate ratio of MFC1 and MFC2 while maintaining a total flow rate of 10 sccm. For example, the 16% n-hexane mixed gas was prepared by transporting the headspace gas from a vial containing n-hexane to the mixing chamber via MFC1 set to 1.6 sccm, and then diluting it with pure nitrogen supplied from MFC2 set to 8.4 sccm, resulting in a flow rate of 10 sccm.

[0058] As shown in Figure 7A, measurements in the frequency domain showed that as the n-hexane concentration increased, the density ρ of the mixed gas increased, resulting in a decrease in the speed of sound c. Here, the six graphs in Figure 7A, from left to right with respect to the peak position, represent n-hexane concentrations of 20%, 16%, 13%, 10%, 8%, and 6% (i.e., descending order of n-hexane concentration). In the time domain measurements shown in Figure 7B, the shape of the microphone output voltage curve changed depending on the n-hexane concentration in the target gas. Specifically, between n-hexane concentrations of 6 and 10%, the output voltage reached its maximum value at the end of the sampling step, but above n-hexane concentrations of 10%, it showed slightly different behavior, reaching its maximum value early in the sampling step and then slowly decreasing toward a steady state. Dynamic turbulence in the headspace within the vial is considered to be the cause of this apparent phenomenon at different concentration levels where the vapor pressure changes. The six graphs in Figure 7B are arranged in descending order of n-hexane concentration (20%, 16%, 13%, 10%, 8%, and 6%) relative to the change in output voltage.

[0059] Figure 7C shows the relationship between the amplitude level (ΔV) of the microphone output voltage and the n-hexane concentration C in the target gas, demonstrating strong linearity (the zero point represents pure nitrogen without n-hexane, i.e., C=0%). Thus, the linearity between ΔV and C can be used to determine the concentration of a chemical substance in the target gas. The ΔV value was acquired at 1440 seconds for comparison (as mentioned above, data at a concentration of 20% was excluded because signal clipping from the microphone occurred with n-hexane). Furthermore, this linearity indicates that the gas used in this experiment can be treated as an ideal gas, and supports the fact that the system used for the measurement operates within the linear acoustic range.

[0060] Figure 7D shows the results of PCA analysis using multidimensional data in the frequency domain (speed of sound) and time domain (see the explanation of Figure 6E above for a description of the features). This shows that different concentration levels of n-hexane in the target gas can be distinguished using these features, demonstrating the advantages of a dual-domain method (measurements in both frequency and time domains) based on multidimensional data.

[0061] [Conclusion] In this embodiment, a quarter-wavelength acoustic resonator was used to measure the characteristics of different gases in both the frequency and time domains. When the target gas was introduced into the resonator, the velocity of sound decreased and the gas density increased compared to the gas before introduction (pure nitrogen supplied in the purge step). As a result, the frequency response shifted in the frequency domain and the sound pressure increased in the time domain. Measurements in the frequency domain (pink noise tests) allowed us to accurately obtain the frequency response functions and sound velocity of different target gases, and from these, we were able to measure the density ρ of each target gas. In the time-domain measurements, a cycle was applied in which sampling and purging of different target gases were repeated at a fixed frequency. A major feature of time-domain analysis is that it is possible to extract multidimensional data. Principal component analysis using the multidimensional data extracted from the time-domain measurements and the sound velocity obtained from the frequency-domain measurements demonstrated that the type of target gas and the attributes of specific gases contained within it at different concentrations could be easily determined. [Industrial applicability]

[0062] The acoustic approach according to the present invention, as described above, has high applicability to a wide range of applications, given that it enables the miniaturization and cost reduction of various acoustic amplifiers and measuring devices, in addition to the recent advancements in low-cost and high-sensitivity MEMS microphones. It is believed that this approach will enable the identification and differentiation of individual chemical species in mixed gases in the future, utilizing existing abundant acoustic devices and signal processing technologies.

[0063] Furthermore, according to the present invention, by using a relatively small acoustic resonator, it is expected that a new acoustic measurement technology system will be created for realizing gas sensing using sound and artificial olfaction. [Explanation of Symbols]

[0064] 100 acoustic resonator 110 circuit boards 112 holes (orifices) 120 Upper enclosure 122, 124 Sealing component (O-ring) 126, 128 openings 130 Lower enclosure 132. Sealing component (O-ring) 140 Sound pressure measurement method (microphone) 142 Sound pressure receiving part (vibrating membrane) 150 Acoustic wave output means (speaker) 152 Acoustic wave generating section (vibrating membrane) C Measurement Chamber

Claims

1. In a method for extracting gas characteristics using acoustic measurements, The casing and An acoustic wave output means attached to one end of the housing, Sound pressure measuring means arranged inside the housing and Using an acoustic resonator having, The steps include: introducing a target gas into the acoustic resonator while emitting an acoustic wave excited by a specific signal from the acoustic wave output means into the acoustic resonator, and measuring the sound pressure caused by the acoustic wave in both the frequency domain and the time domain using the sound pressure measuring means; A step of extracting the characteristics of the target gas from the results obtained in the measurement step, Methods that include...

2. In the measurement step, The measurement in the frequency domain includes measuring the signal output from the sound pressure measuring means using an acoustic wave excited by 1 / f noise with constant energy per octave, and obtaining the frequency response function of the sound pressure acting on the sound pressure measuring means. The measurement in the time domain includes measuring the time variation of the signal output from the sound pressure measuring means using an acoustic wave excited by a fixed-frequency sine wave. The method according to claim 1.

3. The extraction step described above is: This includes obtaining the physical parameters of the target gas from the frequency response function obtained from the measurement in the aforementioned frequency domain, This includes extracting feature quantities from the time evolution of the output signal obtained by the measurement in the aforementioned time domain, The method according to claim 2.

4. The measurement in the aforementioned frequency domain is performed as follows: The signal output from the sound pressure measuring means when the acoustic resonator is filled with the target gas is measured, and a first frequency response function is obtained. The signal output from the sound pressure measuring means when the acoustic resonator is filled with purge gas is measured, and a second frequency response function is obtained. Includes, In the extraction step, the physical parameters of the target gas are obtained based on the selected resonance peak obtained by comparing the first and second frequency response functions. The method according to claim 3.

5. The measurement in the time domain includes introducing the target gas and the purge gas alternately into the acoustic resonator and measuring the time change of the signal output from the sound pressure measuring means. In the extraction step, feature quantities are extracted from the time change of the output signal. The method according to claim 3.

6. The method according to claim 3, further comprising applying principal component analysis or linear discriminant analysis to a data set obtained in the extraction step, which consists of physical parameters of the target gas from measurements in the frequency domain and feature quantities from measurements in the time domain.

7. The method according to any one of claims 1 to 6, wherein the acoustic resonator is a single-ended tube with one end being an open end and the other end being a closed end, and is a resonator with a wavelength of (1 / 4 + 2n / 4) wavelengths (where n is an integer greater than or equal to 0), or a double-ended tube with both ends being open ends, and is a resonator with a wavelength of (1 / 2 + 2n / 2) wavelengths (where n is an integer greater than or equal to 0).

8. The method according to claim 7, wherein the acoustic resonator is the one-sided open tube and is a resonator with a wavelength of (1 / 4 + 2n / 4) wavelengths (where n is an integer greater than or equal to 0).

9. The method according to claim 8, wherein the acoustic resonator is a quarter-wavelength resonator.

10. An apparatus for extracting gas characteristics by acoustic measurement, The casing and An acoustic wave output means attached to one end of the housing, Sound pressure measuring means arranged inside the housing and Equipped with an acoustic resonator having, The aforementioned acoustic resonator has an introduction section for introducing the target gas into its interior, and an exhaust section for discharging the internal gas to the outside. The system is configured such that an acoustic wave excited by a specific signal is emitted from the acoustic wave output means into the acoustic resonator, while a target gas is introduced into the acoustic resonator, and the sound pressure due to the acoustic wave can be measured by the sound pressure measuring means in both the frequency domain and the time domain. Device.

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