Frequency-adjustable photoacoustic spectrophone and gas detection device

Abstract

The invention relates to the technical field of photoacoustic spectrometry, and discloses a frequency-adjustable photoacoustic spectrometry sound detector and a gas detection device.The sound detector comprises an acoustic resonant structure and a quartz tuning fork, the acoustic resonant structure is arranged at one end of the quartz tuning fork, and an acoustic resonant cavity is formed in the acoustic resonant structure in a penetrating mode; the axis of the acoustic resonant cavity is vertical to the length direction of the quartz tuning fork; the quartz tuning fork comprises two vibrating arms, and a vibrating arm gap and a through hole are formed between the two vibrating arms; the two vibrating arms have the same structure, the length of each vibrating arm is L, the width of each vibrating arm is H, and the thickness of each vibrating arm is B; frequency modulation grooves are formed in the outer sides of the bottoms of the two vibration arms and extend in the length direction of the quartz tuning fork, the length of each frequency modulation groove is l, the width of each frequency modulation groove is h, and the thickness of each frequency modulation groove is b; the resonant frequency f, the vibration arm and the frequency modulation groove accord with a specific relational expression. According to the invention, the problem that the traditional QEPAS technology is insufficient in sensitivity when detecting low-relaxation-rate gas molecules such as CH4 and CO is solved, and the application range of the photoacoustic spectrometry technology in the field of gas detection is widened.

Description

Technical Field

[0001] The present invention relates to the technical field of photoacoustic spectroscopy, and particularly to a tunable photoacoustic spectroscopy microphone and a gas detection device. Background Art

[0002] Photoacoustic spectroscopy (PAS) is a detection method based on the photoacoustic effect, which is widely used in gas detection, environmental monitoring, etc., and uses the detected acoustic wave signal to obtain information about the measured substance.

[0003] Quartz-enhanced photoacoustic spectroscopy (QEPAS) uses a quartz crystal oscillator as a converter from acoustic waves to electrical signals. In order to further improve the detection sensitivity, an acoustic resonator is subsequently designed to match the natural resonance frequency of the acoustic resonator with that of the quartz tuning fork, thereby amplifying the acoustic wave signal detected by the tuning fork.

[0004] Traditional QEPAS systems generally use standard commercial quartz tuning forks, and the resonance frequencies of these tuning forks are fixed, which cannot meet the flexibility requirements in specific applications. Specifically, the fixed resonance frequency (about 32 kHz) of the commercial quartz tuning fork requires the modulation frequency of the acoustic wave excitation light source to match it. When measuring low-relaxation-rate gas molecules such as CH4 and CO, the relaxation time caused by the characteristic absorption of these gas molecules does not match the resonance frequency of the commercial quartz tuning fork, resulting in a problem of low detection sensitivity. Summary of the Invention

[0005] The technical problem to be solved by the present invention is that when measuring low-relaxation-rate gas molecules such as CH4 and CO, the relaxation time caused by the characteristic absorption of these gas molecules does not match the resonance frequency of the commercial quartz tuning fork, resulting in a problem of low detection sensitivity.

[0006] In order to solve the above technical problem, the present invention provides a technical solution for a tunable photoacoustic spectroscopy microphone:

[0007] The tunable photoacoustic spectroscopy microphone includes an acoustic resonance structure and a quartz tuning fork. The acoustic resonance structure is arranged at one end of the quartz tuning fork along the length direction. An acoustic resonance cavity is formed through the interior of the acoustic resonance structure, and the axis direction of the acoustic resonance cavity is perpendicular to the length direction of the quartz tuning fork.

[0008] The quartz tuning fork includes a main body portion and two vibrating arms. The two vibrating arms are respectively fixedly connected to the main body portion. There is a vibrating arm gap and a through hole between the two vibrating arms. The axis of the through hole is perpendicular to the length direction of the quartz tuning fork and communicates with the vibrating arm gap. The acoustic resonance structure is installed in the through hole, and pins are also provided at the end of the main body portion away from the acoustic resonance structure.

[0009] The structures of the two vibrating arms are the same. The length of the vibrating arm is L, the width is H, and the thickness is B. On the outer sides of the bottoms of the two vibrating arms, frequency modulation grooves are provided. The frequency modulation grooves extend along the length direction of the quartz tuning fork. The length of the frequency modulation groove is l, the width is h, and the thickness is b.

[0010] The resonant frequency f of the quartz tuning fork and the dimensions of the vibrating arm and the frequency modulation groove satisfy the following relational expression:

[0011]

[0012] Furthermore, it is known that the Young's modulus of the quartz material is E and the density is ρ.

[0013] Among them, the moment of inertia of the cross-section of the vibrating arm is I, and the formula for the moment of inertia of the vibrating arm is:

[0014] The correction value of the moment of inertia of the vibrating arm by the frequency modulation groove is I eff , and the correction formula for the moment of inertia of the vibrating arm by the frequency modulation groove is:

[0015]

[0016] The correction value of the cross-section of the vibrating arm by the frequency modulation groove is A eff , and the correction formula for the cross-section of the vibrating arm by the frequency modulation groove is:

[0017] The resonant frequency f of the quartz tuning fork and I eff and A eff satisfy the following relational expression:

[0018]

[0019] The formula for the resonant frequency f of the quartz tuning fork is:

[0020] Furthermore, the value range of the length l of the frequency modulation groove is from 0.05 mm to 0.5 mm, the value range of the width h of the frequency modulation groove is from 0.03 mm to 0.18 mm, and the value range of the thickness b of the frequency modulation groove is from 0.02 mm to 0.2 mm.

[0021] Furthermore, the value range of the width of the vibrating arm gap is from 200 μm to 400 μm. The aperture of the through hole is larger than the width of the vibrating arm gap, and the distance range between the center of the through hole and the top end of the quartz tuning fork is from 0 to 3 mm.

[0022] Furthermore, the through hole is a circular through hole, the diameter of the through hole is D, and the effective mass of the vibrating arm is m eff, the effective mass of the vibrating arm and the diameter of the through hole satisfy the following relational expression:

[0023]

[0024] The resonance frequency f of the quartz tuning fork and the effective mass of the vibrating arm satisfy the following relational expression:

[0025]

[0026] Furthermore, the resonance frequency f of the quartz tuning fork and the through hole satisfy the following relational expression:

[0027]

[0028] Furthermore, the resonance frequency f of the quartz tuning fork and the vibrating arm, the frequency modulation groove, and the through hole satisfy the following relational expression:

[0029]

[0030] Furthermore, the inner diameter of the acoustic resonance cavity ranges from 0.2 mm to 0.5 mm, and the outer diameter of the acoustic resonance cavity is smaller than the diameter of the through hole.

[0031] Furthermore, the shape of the acoustic resonance cavity is cylindrical, the central axis of the acoustic resonance cavity extends along the hole center line of the through hole, and the gap between the acoustic resonance cavity and the inner wall of the through hole ranges from 0 to 0.08 mm.

[0032] To solve the above technical problems, the present invention provides a technical solution for a gas detection device:

[0033] The gas detection device includes a tunable photoacoustic spectroscopy microphone, as well as a laser and a signal demodulator. The laser is used to generate a laser beam into the tunable photoacoustic spectroscopy microphone to stimulate the photoacoustic effect of the molecules in the gas to be detected; the signal demodulator is electrically connected to the tunable photoacoustic spectroscopy microphone to demodulate the electrical signal generated by the quartz tuning fork;

[0034] The tunable photoacoustic spectroscopy microphone includes an acoustic resonance structure and a quartz tuning fork. The acoustic resonance structure is arranged at one end of the quartz tuning fork along the length direction. An acoustic resonance cavity is formed through the interior of the acoustic resonance structure, and the axis direction of the acoustic resonance cavity is perpendicular to the length direction of the quartz tuning fork;

[0035] The quartz tuning fork includes a main body portion and two vibrating arms. The two vibrating arms are respectively fixedly connected to the main body portion. There are a vibrating arm gap and a through hole between the two vibrating arms. The axis of the through hole is perpendicular to the length direction of the quartz tuning fork and communicates with the vibrating arm gap. The acoustic resonance structure is installed in the through hole. A lead is also provided at the end of the main body portion away from the acoustic resonance structure.

[0036] The structures of the two vibrating arms are the same. The length of the vibrating arm is L, the width is H, and the thickness is B. Frequency modulation grooves are provided on the outer sides of the bottoms of the two vibrating arms. The frequency modulation grooves extend along the length direction of the quartz tuning fork. The length of the frequency modulation groove is l, the width is h, and the thickness is b.

[0037] The resonant frequency f of the quartz tuning fork and the dimensions of the vibrating arm and the frequency modulation groove satisfy the following relational expression:

[0038]

[0039] Further, the Young's modulus of the quartz material is E and the density is ρ.

[0040] Wherein, the moment of inertia of the cross-section of the vibrating arm is I, and the formula for the moment of inertia of the vibrating arm is:

[0041] The correction value of the moment of inertia of the vibrating arm by the frequency modulation groove is I eff , and the correction formula for the moment of inertia of the vibrating arm by the frequency modulation groove is:

[0042]

[0043] The correction value of the cross-section of the vibrating arm by the frequency modulation groove is A eff , and the correction formula for the cross-section of the vibrating arm by the frequency modulation groove is:

[0044] The resonant frequency f of the quartz tuning fork and I eff and A eff satisfy the following relational expression:

[0045]

[0046] The formula for the resonant frequency f of the quartz tuning fork is:

[0047] Further, the value range of the length l of the frequency modulation groove is 0.05 mm to 0.5 mm, the value range of the width h of the frequency modulation groove is 0.03 mm to 0.18 mm, and the value range of the thickness b of the frequency modulation groove is 0.02 mm to 0.2 mm.

[0048] Further, the width of the vibrating arm gap ranges from 200 μm to 400 μm, the aperture of the through hole is larger than the width of the vibrating arm gap, and the distance between the center of the through hole and the top end of the quartz tuning fork ranges from 0 to 3 mm.

[0049] Further, the through hole is a circular through hole, the diameter of the through hole is D, and the effective mass of the vibrating arm is m eff , and the effective mass of the vibrating arm and the diameter of the through hole satisfy the following relational expression:

[0050]

[0051] The resonant frequency f of the quartz tuning fork and the effective mass of the vibrating arm satisfy the following relational expression:

[0052]

[0053] Further, the resonant frequency f of the quartz tuning fork and the through hole satisfy the following relational expression:

[0054]

[0055] Further, the resonant frequency f of the quartz tuning fork and the vibrating arm, the frequency modulation groove and the through hole satisfy the following relational expression:

[0056]

[0057] Further, the inner diameter of the acoustic resonant cavity ranges from 0.2 mm to 0.5 mm, and the outer diameter of the acoustic resonant cavity is smaller than the diameter of the through hole.

[0058] Further, the shape of the acoustic resonant cavity is cylindrical, the central axis of the acoustic resonant cavity extends along the hole center line of the through hole, and the gap between the acoustic resonant cavity and the inner wall of the through hole ranges from 0 to 0.08 mm.

[0059] Further, it further includes a function generator, an adder, a laser driver, a laser, an optical fiber collimating lens, and a computer device; the modulation signal output end of the function generator is sequentially connected to the adder, the laser driver, and the laser; the synchronization signal output end of the function generator is connected to the signal demodulator; the computer device is further respectively connected to the output end of the signal demodulator and the input end of the function generator.

[0060] Compared with the prior art, an adjustable frequency photoacoustic spectroscopy microphone and a gas detection device of the present invention have the following beneficial effects: The principle of the adjustable frequency photoacoustic spectroscopy microphone for testing gas concentration is as follows: An acoustic resonator is arranged at one end of a quartz tuning fork, and the resonance frequency of the acoustic resonator is the same as that of the quartz tuning fork. During the test, a laser beam is incident from the side of the acoustic resonator far from the quartz tuning fork, passes through the acoustic resonator and exits from the other end of the acoustic resonator, and finally passes through the quartz tuning fork. In this process, the laser beam excites the photoacoustic effect of the molecules in the gas to be measured, and the gas to be measured generates an acoustic wave signal. Then, the acoustic wave signal drives the quartz tuning fork to vibrate to form an electrical signal, and the intensity of the acoustic wave signal generated by the gas to be measured is related to the concentration of the gas to be measured. Therefore, the concentration information of the gas to be measured can be finally obtained by analyzing the electrical signal.

[0061] The effective moment of inertia of the quartz tuning fork is related to the cube of the width h of the frequency modulation groove 3 That is, the effective moment of inertia of the quartz tuning fork is extremely sensitive to the change in the width h of the frequency modulation groove, while the frequency modulation groove only has a linear effect on the cross-sectional area of the vibrating arm, and the change amount is small. When the width h increases, due to the sensitivity of the moment of inertia to the width h, the frequency of the quartz tuning fork decreases significantly; when the thickness b increases, the moment of inertia is linearly reduced, and the influence on the frequency of the quartz tuning fork is secondary; when the length l increases, it plays a role in amplifying the influence of the thickness b and the width h, and further reduces the frequency of the quartz tuning fork.

[0062] Compared with the prior art, the ambient noise is not processed, resulting in a low signal-to-noise ratio and low precision of gas detection. On the other hand, higher requirements are imposed on the collimation and profile quality of the laser beam to obtain relatively good acoustic wave signals. The adjustable frequency design achieved by changing the preset geometric parameters of the frequency modulation groove can adapt to the measurement of gas molecules with different relaxation frequencies, thereby broadening the application range of the QEPAS technology. Especially for gases with low relaxation rates, the present invention provides an effective detection means, ensuring the detection sensitivity and overcoming the limitations of traditional QEPAS systems in the measurement of these gases. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Figure 1 is a three-dimensional schematic diagram of the adjustable frequency photoacoustic spectroscopy microphone in an embodiment of the present invention;

[0064] Figure 2 is a front view schematic diagram of the adjustable frequency photoacoustic spectroscopy microphone in an embodiment of the present invention;

[0065] Figure 3 is a top view schematic diagram of the adjustable frequency photoacoustic spectroscopy microphone in an embodiment of the present invention;

[0066] Figure 4 is a structural block diagram of the gas detection device in an embodiment of the present invention;

[0067] Figure 5 It is a simulation verification relationship diagram of frequency and FM slot width in the embodiment of the present invention;

[0068] Figure 6 It is a simulation verification relationship diagram of frequency and FM slot thickness in the embodiment of the present invention;

[0069] Figure 7 It is a simulation verification relationship diagram of frequency and FM slot length in the embodiment of the present invention;

[0070] Figure 8 It is a comparison diagram of the vibration amplitude of the vibrating arm of the tunable frequency photoacoustic spectrometer for gas molecules with a vibration-translation (VT) relaxation time of 11 μs in the embodiment of the present invention and the existing tuning fork;

[0071] Figure 9 It is a comparison diagram of the vibration amplitude of the vibrating arm of the tunable frequency photoacoustic spectrometer for gas molecules with a vibration-translation (VT) relaxation time of 30.89 μs in the embodiment of the present invention and the existing tuning fork;

[0072] Figure 10 It is a comparison diagram of the vibration amplitude of the vibrating arm of the tunable frequency photoacoustic spectrometer for gas molecules with a vibration-translation (VT) relaxation time of 61.67 μs in the embodiment of the present invention and the existing tuning fork;

[0073] Figure 11 It is a comparison diagram of the vibration amplitude of the vibrating arm of the tunable frequency photoacoustic spectrometer for gas molecules with a vibration-translation (VT) relaxation time of 110 μs in the embodiment of the present invention and the existing tuning fork;

[0074] In the figure: 1 - acoustic resonance structure, 10 - acoustic resonance cavity, 11 - laser beam, 12 - tunable frequency photoacoustic spectrometer, 2 - quartz tuning fork, 20 - main body, 21 - vibrating arm, 22 - vibrating arm gap, 23 - through hole, 24 - pin, 25 - FM slot, 3 - computer device, 4 - function generator, 5 - adder, 6 - laser, 60 - laser driver, 7 - fiber collimating lens, 8 - preamplifier, 9 - lock-in amplifier. Specific Embodiments

[0075] The following combines the drawings and embodiments to further describe in detail the specific embodiments of the present invention. The following embodiments are used to illustrate the present invention, but are not used to limit the scope of the present invention.

[0076] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by the terms "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", etc. in the present invention is based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation to the present invention.

[0077] In addition, the terms "first" and "second" are only used for descriptive purposes and should not be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the present invention, the meaning of "a plurality" is two or more, unless otherwise specifically defined.

[0078] In the present invention, unless otherwise clearly specified and limited, the terms "mounted", "connected", "coupled", "fixed", etc. should be understood in a broad sense. For example, it may be a fixed connection, a detachable connection, or integrated; it may be directly connected or indirectly connected through an intermediate medium, and it may be the internal communication of two elements or the interaction relationship between two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0079] As Figures 1 to 3 shown, an adjustable frequency photoacoustic spectroscopy microphone according to an embodiment of the present invention includes an acoustic resonance structure 1 and a quartz tuning fork 2. The acoustic resonance structure 1 is provided at one end of the quartz tuning fork 2 along the length direction. An acoustic resonance cavity 10 is formed through the interior of the acoustic resonance structure 1, and the axis direction of the acoustic resonance cavity 10 is perpendicular to the length direction of the quartz tuning fork 2. The quartz tuning fork 2 includes a main body portion 20 and two vibrating arms 21. The two vibrating arms 21 are respectively fixedly connected to the main body portion 20. A gap 22 and a through hole 23 are provided between the two vibrating arms 21. The axis of the through hole 23 is perpendicular to the length direction of the quartz tuning fork 2 and communicates with the gap 22. The acoustic resonance structure 1 is installed in the through hole 23. A lead 24 is further provided at the end of the main body portion 20 away from the acoustic resonance structure 1.

[0080] The structures of the two vibrating arms 21 are the same. The length of the vibrating arm 21 is L, the width is H, and the thickness is B. On the outer sides of the bottoms of the two vibrating arms 21, frequency modulation grooves 25 are provided. The frequency modulation grooves 25 extend along the length direction of the quartz tuning fork 2. Here, the "bottom" refers to the position of the vibrating arm 21 away from the acoustic resonator 10 and close to the main body 20. The length of the frequency modulation groove 25 is l, the width is h, and the thickness is b. The resonance frequency f of the quartz tuning fork 2 and the dimensions of the vibrating arm 21 and the frequency modulation groove 25 satisfy the following relational expression:

[0081]

[0082] The principle of the tunable photoacoustic spectroscopy microphone 12 for testing gas concentration is as follows: The acoustic resonator 10 is arranged at one end of the quartz tuning fork 2, and the resonance frequency of the acoustic resonator 10 is the same as that of the quartz tuning fork 2. During the test, the laser beam 11 is incident from the side of the acoustic resonator 10 away from the quartz tuning fork 2, passes through the acoustic resonator 10 and exits from the other end of the acoustic resonator 10, and finally passes through the quartz tuning fork 2. In this process, the laser beam 11 excites the photoacoustic effect of the molecules in the gas to be measured, the gas to be measured generates an acoustic wave signal, and then the acoustic wave signal pushes the quartz tuning fork 2 to vibrate to form an electrical signal. Moreover, the intensity of the acoustic wave signal generated by the gas to be measured is related to the concentration of the gas to be measured. Therefore, the concentration information of the gas to be measured can be finally obtained by analyzing the electrical signal.

[0083] The effective moment of inertia of the quartz tuning fork 2 is associated with the cube of the width h of the frequency modulation groove 25 3 That is, the effective moment of inertia of the quartz tuning fork 2 is extremely sensitive to the change in the width h of the frequency modulation groove 25, while the frequency modulation groove 25 only has a linear influence on the cross-sectional area of the vibrating arm 21, and the change amount is small. When the width h increases, due to the sensitivity of the moment of inertia to the width h, the frequency of the quartz tuning fork 2 decreases significantly; when the thickness b increases, the moment of inertia is linearly reduced, and the influence on the frequency of the quartz tuning fork 2 is secondary; when the length l increases, it plays a role in amplifying the influence of the thickness b and the width h, and further reduces the frequency of the quartz tuning fork 2.

[0084] Compared with the prior art, the surrounding environmental noise is not processed, the signal-to-noise ratio is low, resulting in low accuracy of gas detection. On the other hand, the requirements for the collimation and profile quality of the laser beam 11 are relatively high to obtain a relatively good acoustic wave signal. The tunable frequency design realized by changing the preset geometric parameters of the frequency modulation groove can adapt to the measurement of gas molecules with different relaxation frequencies, thereby broadening the application range of the QEPAS technology. Especially for gases with low relaxation rates, the present invention provides an effective detection means, ensuring the detection sensitivity and overcoming the limitations of traditional QEPAS systems in the measurement of these gases.

[0085] In this embodiment, the Young's modulus of the quartz material is known to be E = 72 GPa, and the density is ρ = 2650 kg / m 3 ;

[0086] Among them, the moment of inertia of the cross-section of the vibrating arm 21 is I, and the formula for the moment of inertia of the vibrating arm is:

[0087] The influence of the frequency modulation groove 25 needs to be corrected according to the ratio of its length l to the total length L of the vibrating arm. The correction value of the moment of inertia of the vibrating arm by the frequency modulation groove 25 is I eff , and the correction formula for the moment of inertia of the vibrating arm by the frequency modulation groove 25 is:

[0088]

[0089] The correction value of the cross-section of the vibrating arm by the frequency modulation groove 25 is A eff , and the correction formula for the cross-section of the vibrating arm by the frequency modulation groove 25 is:

[0090] The resonant frequency f of the quartz tuning fork 2 is related to I eff and A eff and satisfies the following relationship:

[0091]

[0092] The formula for the resonant frequency f of the quartz tuning fork 2 is:

[0093] The quartz tuning fork 2 is a cantilever beam fixed at one end and free at the other end, which conforms to the Euler-Bernoulli beam theory, that is, it satisfies the relationship of the correction formula for the moment of inertia of the vibrating arm by the frequency modulation groove 25. Among them, the value range of the length l of the frequency modulation groove 25 is 0.05 mm to 0.5 mm, the value range of the width h of the frequency modulation groove 25 is 0.03 mm to 0.18 mm, and the value range of the thickness b of the frequency modulation groove 25 is 0.02 mm to 0.2 mm. As a further preferred solution, the diameter D of the through hole 23 can be selected as 0.3 mm, the thickness b of the frequency modulation groove 25 is 0.2 mm, the width h of the frequency modulation groove 25 is 0.18 mm, and the length l of the frequency modulation groove 25 is 0.45 mm, and the frequency f of the quartz tuning fork 2 with the frequency modulation function is 16 kHz.

[0094] Comparing the quartz tuning fork 2 of this embodiment with the existing tuning fork (32 kHz) for analysis, as Figures 8 to 11 shown, when detecting four molecules with different relaxation times, the vibration amplitude of the tuning fork vibrating arm obtained by the finite element analysis method. Here, since the vibration amplitude of the tuning fork vibrating arm is positively correlated with the magnitude of its output electrical signal, the vibration amplitude of the tuning fork vibrating arm is used to represent the magnitude of its signal.

[0095] With other experimental conditions remaining unchanged, when detecting molecules with a detection relaxation time of 11 μs, the vibration amplitude of the existing tuning fork vibrating arms is greater than that of a 16 kHz frequency-modulated tuning fork; when detecting molecules with a detection relaxation time of 30.89 μs, the vibration amplitude of the existing tuning fork vibrating arms is comparable to that of a 16 kHz frequency modulation; while when detecting molecules with a detection relaxation time of 61.67 μs or 110 μs, the vibration amplitude of the 16 kHz frequency-modulated tuning fork vibrating arms is significantly higher than that of the existing tuning fork. Thus, it can be seen that for detecting low-relaxation-rate gases, a 16 kHz frequency-modulated quartz tuning fork can generate a larger piezoelectric signal.

[0096] In this embodiment, the width of the vibrating arm gap 22 ranges from 200 μm to 400 μm, the aperture of the through hole 23 is larger than the width of the vibrating arm gap 22, and the distance between the center of the through hole 23 and the top end of the quartz tuning fork 2 ranges from 0 to 3 mm. Specifically, the through hole 23 is a circular through hole, the diameter of the through hole 23 is D, and the effective mass of the vibrating arm 21 is m eff , and the effective mass of the vibrating arm 21 and the diameter of the through hole 23 satisfy the following relationship:

[0097]

[0098] The resonant frequency f of the quartz tuning fork 2 and the effective mass of the vibrating arm 21 satisfy the following relationship:

[0099]

[0100] The resonant frequency f of the quartz tuning fork 2 and the through hole 23 satisfy the following relationship:

[0101]

[0102] It can be seen from the above formula that as the diameter D of the round hole increases, the effective mass m eff will decrease, and this change will in turn cause the resonant frequency of the quartz tuning fork 2 to rise. Therefore, it can be obtained that the design of the through hole 23 can effectively increase the resonant frequency of the quartz tuning fork 2. Using the design of this quartz tuning fork 2, including the vibrating arm gap 22, the through hole 23, and the frequency modulation groove 25, not only optimizes the resonant frequency of the quartz tuning fork 2, but also improves its sensitivity and stability to acoustic wave signals. Especially the design of the frequency modulation groove 25 enables the resonant frequency of the quartz tuning fork 2 to be finely tuned within a certain range to adapt to different measurement conditions.

[0103] The resonant frequency f of the quartz tuning fork 2 and the vibrating arm 21, the frequency modulation groove 25, and the through hole 23 satisfy the following relationship:

[0104]

[0105] The inner diameter of the acoustic resonator 10 ranges from 0.2 mm to 0.5 mm. The outer diameter of the acoustic resonator 10 is smaller than the diameter of the through hole 23. The length of the acoustic resonator 10 is calculated and determined by the acoustic wavelength so that the resonance frequency of the acoustic resonator 10 is the same as the resonance frequency of the quartz tuning fork 2. It should be noted that the quartz tuning fork 2 is optimized with a cylindrical crystal oscillator having a resonance frequency of 32.768 kHz. The shape of the acoustic resonator 10 is cylindrical, and the central axis of the acoustic resonator 10 extends along the center of the hole of the through hole 23. The gap between the acoustic resonator 10 and the inner wall of the through hole 23 ranges from 0 to 0.08 mm to ensure that the laser beam 11 can accurately pass through the through hole 23 and excite the acoustic signal. Moreover, the laser beam 11 is coaxial with the acoustic resonator 10, and the waist diameter of the laser beam 11 should be smaller than the diameter D of the through hole 23.

[0106] First, the cross-sectional areas of the through hole 23 of the quartz tuning fork 2 and the acoustic resonator 10 are larger, thereby reducing the collimation difficulty of the laser. For the laser to better enter the acoustic resonator 10, the acoustic resonator 10 can also guide the laser beam 11. At the same time, the quality requirements for the outer contour of the laser beam 11 are also reduced. The frequency modulation groove 25 of the quartz tuning fork 2 can control the low-order resonance frequency range of the quartz tuning fork 2 from 24 kHz to 40 kHz, enabling the tunable photoacoustic spectrometer 12 to match more light sources.

[0107] Second, the acoustic resonator 10 can enhance the sound pressure near the vibrating arms of the quartz tuning fork 2 and suppress environmental noise, thereby improving the signal-to-noise ratio of the quartz tuning fork 2 for detecting acoustic signals near its vibrating arms, and further improving the gas detection accuracy.

[0108] Specifically, when the laser beam 11 passes through the gas to be measured, it will excite the molecules in the gas to be measured to generate acoustic signals. After these acoustic signals are resonantly enhanced in the acoustic resonator 10, they will push the two vibrating arms 21 of the quartz tuning fork 2 to vibrate, and the electrical signals generated by the vibration will be transmitted to the signal processing module through the pins 24 for subsequent processing. The vibration generator and the signal processing module are both used to measure the crystal oscillation frequency of the quartz tuning fork 2. One of the pins 24 is electrically connected to the vibration generator, and the other pin 24 is electrically connected to the signal processing module.

[0109] Specifically, the quartz tuning fork 2 uses a strongly resonant quartz tuning fork of standard size. In a specific embodiment, the widths, thicknesses, and the size of the gap between the vibrating arms of the quartz tuning fork 2 are 600 μm, 330 μm, and 300 μm respectively. A circular through hole is provided between the vibrating arms of the quartz tuning fork 2, and the diameter of the circular through hole is 1 mm. By changing the lengths, widths, and thicknesses of the frequency modulation grooves 25 on both sides of the vibrating arms of the quartz tuning fork 2, the low-order resonance frequency of the quartz tuning fork 2 can be controlled.

[0110] In a specific embodiment, the length of the acoustic resonator 10 ranges from 4 mm to 5 mm. Specifically, the length of the acoustic resonator 10 is 4.6 mm. The inner wall of the acoustic resonator 10 and the surface of the quartz tuning fork 2 are both set as hard sound field boundaries, so that total reflection of sound waves can occur. Since the sound waves generated by photoacoustic spectroscopy are cylindrically distributed around the laser beam 11, and the cylinder of the sound wave is coaxial with the cylinder of the beam, the sound waves generated by the laser beam 11 can be set as a cylindrical sound source with the sound source intensity remaining unchanged. Currently, the optimal length of the cylindrical acoustic resonator 10 is 4.6 mm. By changing the inner radius of the acoustic resonator 10, the signal-to-noise ratio can be reduced and the gas detection accuracy can be improved.

[0111] After passing through the acoustic resonator 10, the laser beam 11 will continue to pass through the circular opening of the quartz tuning fork 2. In order to ensure that the laser beam 11 can pass through the circular opening smoothly, when the laser beam 11 is coaxial with the acoustic resonator 10, the axis of the acoustic resonator 10 also needs to be perpendicular to the central axis of the quartz tuning fork 2 and intersect, and the length of the widest part of the laser beam 11 should also be less than the circular opening, that is, the beam waist radius of the laser beam 11 should be less than the circular opening, so as to facilitate the laser beam 11 to pass through the quartz tuning fork 2 smoothly.

[0112] Furthermore, as Figures 5 to 7 shown, finite element simulation can be carried out through simulation software to assist in analyzing the length, width, and thickness of the frequency modulation slot 25 of the quartz tuning fork 2. By parametrically scanning the width h, thickness b, and length l of the frequency modulation slot from 0.03 mm to 0.18 mm, 0.02 mm to 0.2 mm, and 0.05 mm to 0.5 mm respectively, the low-order resonance frequency of the quartz tuning fork under different sizes of the frequency modulation slot is simulated, and finally it is obtained that under the condition that the size of the frequency modulation slot becomes larger, its low-order resonance frequency is significantly reduced.

[0113] Preferably, the acoustic resonance structure 1 is made of stainless steel material.

[0114] As Figure 4 shown, the present invention also provides a gas detection device, including a tunable photoacoustic spectroscopy microphone 12. The vibrating arms 21 of the quartz tuning fork 2 are arranged in the gas to be measured. It also includes a laser 6 and a signal demodulator. The laser 6 is used to generate a laser beam 11 into the tunable photoacoustic spectroscopy microphone 12 to stimulate the photoacoustic effect of the molecules in the gas to be measured; the signal demodulator is electrically connected to the tunable photoacoustic spectroscopy microphone 12 to demodulate the electrical signal generated by the quartz tuning fork 2.

[0115] Further, the signal demodulator includes a preamplifier 8 and a lock-in amplifier 9. The preamplifier 8 is electrically connected to the quartz tuning fork 2 and the lock-in amplifier 9 respectively. The preamplifier 8 is used to perform transimpedance amplification on the electrical signal generated by the quartz tuning fork 2 to obtain a photoacoustic signal. The lock-in amplifier 9 is used to demodulate the photoacoustic signal output by the preamplifier 8 to obtain the concentration information of the gas to be measured.

[0116] Furthermore, the gas detection device further includes a function generator 4, an adder 5, a laser driver 60, an optical fiber collimating lens 7, and a computer device 3. The modulation signal output terminal of the function generator 4 is sequentially connected to the adder 5, the laser driver 60, and the laser 6. The synchronization signal output terminal of the function generator 4 is connected to the signal demodulator. The computer device 3 is also respectively connected to the output terminal of the signal demodulator and the input terminal of the function generator 4.

[0117] Among them, the lock-in amplifier 9 is connected to the synchronization signal output terminal of the function generator 4 and the input terminal of the computer device 3. The lock-in amplifier 9 uses the reference signal of the function generator 4 for demodulation work and inputs the demodulated signal into the computer device 3.

[0118] Specifically, the laser 6 is used to provide a high-energy laser beam 11 to excite the molecules in the gas to be measured. The optical fiber collimating lens 7 is used to shape the laser beam 11 and incident the shaped laser beam 11 into the tunable photoacoustic spectroscopy detector 12. The signal demodulator is connected to the tunable photoacoustic spectroscopy detector 12 to demodulate the electrical signal to obtain the concentration information of the gas to be measured. The laser driver 60 can drive the laser 6. Further, the preamplifier 8 can perform transimpedance amplification on the electrical signal output by the quartz tuning fork 2 to enhance the signal strength and obtain a clear photoacoustic signal. The lock-in amplifier 9 is used to accurately demodulate the photoacoustic signal output by the preamplifier 8 to calculate and obtain the concentration information of the gas to be measured. In addition, the laser 6 emits at least mid-infrared light.

[0119] In the specific detection process, the entire gas detection device adopts harmonic detection technology. A sine wave with a modulation frequency of half of the resonance frequency of the quartz tuning fork 2 generated by the function generator 4 is sent to the laser driver 60. The laser driver 60 can control the injection current and temperature of the laser 6. The laser beam 11 emitted by the laser 6 enters the tunable photoacoustic spectroscopy detector 12 after passing through the fiber collimating lens 7. The laser beam 11 first enters the acoustic resonance cavity 10, then passes through the vibration arm gap of the quartz tuning fork 2, and finally exits. The laser beam 11 emitted by the laser 6 excites the gas to be detected to generate sound waves, and the sound waves push the tuning fork to vibrate, thereby generating an electrical signal and outputting it to the preamplifier 8, and then sending it to the lock-in amplifier 9 for demodulation. The reference signal for demodulation by the lock-in amplifier 9 comes from the synchronous port of the function generator 4. The signal demodulated by the lock-in amplifier 9 is sent to the computer device 3 with a data acquisition card to collect and record data. In addition, the concentration of the gas to be detected measured in real time can also be displayed on a mobile computer, which has the functions of high precision, strong portability, and on-line monitoring.

[0120] When detecting trace gases, first select a target detection line close to the central wavelength of the light source. By controlling the drive current of the laser driver 60, the wavelength of the laser 6 is swept across the target absorption line. Specifically, the second harmonic detection technology is adopted, and the current of the laser 6 is frequency-modulated by the f / 2 signal frequency generated by the function generator 4, where f is the resonance frequency of the tuning fork used. After the gas to be detected is excited by the laser, the acoustic wave signal generated by the de-excitation is collected by the quartz tuning fork 2 and converted into a corresponding electrical signal. The electrical signal passes through the preamplifier 8 and the lock-in amplifier 9 in sequence, and then enters the system of the computer device 3 through the data acquisition card. After the data is calculated by software, the gas concentration information is finally displayed on the screen through a man-machine interactive interface. When measuring a gas with an unknown concentration, it should be calibrated in advance with a standard gas of a known concentration, and the calibrated device can measure this kind of gas.

[0121] In a more specific embodiment, the gas detection device of the sound detector based on the present invention operates as follows: First, a laser 6 is used as an excitation source. Then, a high-precision semiconductor laser 6 driver board is used to control the temperature and injection current of the semiconductor laser 6. The second harmonic wavelength modulation technique is used to improve the detection sensitivity of the quartz tuning fork 2 enhanced spectroscopy technique. A triangular wave with a period of 400 s and a sine wave with a frequency of f0 / 2 (f0 is the resonance frequency of the spectroscopic sound detector) are generated by a signal generator. The second harmonic wavelength modulation technique is used to reduce the influence of background noise due to stray light and crosstalk of other gas absorption lines. The laser beam 11 is focused through an acoustic resonator 10 by a self-focusing lens and then passes through the gap between the two arms of the quartz tuning fork 2. The focal length of the self-focusing lens is 11 mm, and the beam waist diameter of the laser is approximately 100 μm. The electrical signal output by the quartz tuning fork 2 is amplified by a customized 10 MΩ transimpedance preamplifier 8. A lock-in amplifier 9 is used to demodulate the second harmonic signal. Finally, the overall system of the gas detection device is controlled by a program on the computer device 3, and the concentration of the gas is calculated.

[0122] In some embodiments, the emission center wavelength of the laser 6 corresponds to the target absorption line of the gas to be measured. Since the cross-sectional area of the circular through-hole between the vibrating arms of the quartz tuning fork 2 is relatively large, the light collimation space is opened, which further reduces the requirement for the profile quality of the laser beam 11, enabling the tunable frequency photoacoustic spectroscopy sound detector 12 to match and use more light sources, enhancing the flexibility and practicality of its application. The frequency modulation grooves 25 and the acoustic resonator 10 at the bottoms of both sides of the vibrating arms of the quartz tuning fork 2 have relatively more adjustable structural parameters, which is beneficial to adjusting the resonance frequency and bandwidth, and improving the quality factor and stability of the system.

[0123] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the technical principle of the present invention, several improvements and substitutions can be made, and these improvements and substitutions should also be regarded as the protection scope of the present invention.

Claims

1. An adjustable frequency photoacoustic spectrophone, characterized in that, It comprises an acoustic resonance structure and a quartz tuning fork, wherein the acoustic resonance structure is arranged at one end of the quartz tuning fork along the length direction, an acoustic resonance cavity is provided through the interior of the acoustic resonance structure, and the axial direction of the acoustic resonance cavity is perpendicular to the length direction of the quartz tuning fork; The quartz tuning fork comprises a main body and two vibration arms, the two vibration arms are respectively fixedly connected to the main body, a vibration arm gap and a through hole are provided between the two vibration arms, the hole axis of the through hole is perpendicular to the length direction of the quartz tuning fork and is connected to the vibration arm gap, the acoustic resonance structure is installed in the through hole, and a pin is further provided at the end of the main body away from the acoustic resonance structure; The two vibration arms have the same structure, the length of the vibration arm is L, the width of the vibration arm is H, and the thickness of the vibration arm is B; the outer sides of the bottoms of the two vibration arms are provided with frequency modulation grooves, the frequency modulation grooves extend along the length direction of the quartz tuning fork, the length of the frequency modulation grooves is l, the width of the frequency modulation grooves is h, and the thickness of the frequency modulation grooves is b; The resonant frequency f of the quartz tuning fork and the dimensions of the vibration arm and the frequency modulation slot satisfy the following relationship:

2. The tunable frequency photoacoustic spectrophone according to claim 1, wherein The Young's modulus of the quartz material is E and the density is ρ; Among them, the moment of inertia of the cross-section of the vibrating arm is I, and the formula for the moment of inertia of the vibrating arm is: The correction value of the tuning slot for the moment of inertia of the vibrating arm is I eff , and the correction formula of the tuning slot for the moment of inertia of the vibrating arm is as follows: The correction value of the frequency modulation groove to the cross-section of the vibrating arm is A eff , and the correction formula of the frequency modulation groove to the cross-section of the vibrating arm is: The resonance frequency f of the quartz tuning fork and I eff and A eff Satisfy the following relationship: The resonance frequency f formula of the quartz tuning fork is as follows:

3. The tunable frequency photoacoustic spectrophone according to claim 1, wherein, The length l of the frequency modulation slot ranges from 0.05 mm to 0.5 mm, the width h of the frequency modulation slot ranges from 0.03 mm to 0.18 mm, and the thickness b of the frequency modulation slot ranges from 0.02 mm to 0.2 mm.

4. The tunable frequency photoacoustic spectrophone according to claim 1, wherein The width of the gap between the vibration arms ranges from 200 μm to 400 μm, the aperture of the through hole is larger than the width of the gap between the vibration arms, and the distance between the center of the through hole and the top of the quartz tuning fork ranges from 0 to 3 mm.

5. The tunable frequency photoacoustic spectrometer according to claim 1, characterized in that, The through hole is a circular through hole, the diameter of the through hole is D, and the effective mass of the vibrating arm is m eff , and the effective mass of the vibrating arm and the diameter of the through hole satisfy the following relationship: The resonant frequency f of the quartz tuning fork and the effective mass of the vibration arm satisfy the following relationship:

6. The tunable frequency photoacoustic spectrophone according to claim 5, characterized in that, The resonant frequency f of the quartz tuning fork and the through hole satisfy the following relationship:

7. The tunable frequency photoacoustic spectrometer according to claim 6, characterized in that, The resonant frequency f of the quartz tuning fork satisfies the following relationship with the vibration arm, the frequency modulation slot and the through hole:

8. The tunable frequency photoacoustic spectrophone according to claim 1, characterized in that, The inner diameter of the acoustic resonance cavity ranges from 0.2 mm to 0.5 mm, and the outer diameter of the acoustic resonance cavity is smaller than the diameter of the through hole.

9. The tunable frequency photoacoustic spectrophone according to claim 1, characterized in that, The acoustic resonance cavity is cylindrical in shape, the central axis of the acoustic resonance cavity is extended along the center line of the through hole, and the gap between the acoustic resonance cavity and the inner wall of the through hole ranges from 0 to 0.08 mm.

10. A gas detection device, characterized in that, The invention comprises the frequency-adjustable photoacoustic spectrometer as claimed in any one of claims 1 to 9, as well as a laser and a signal demodulator, wherein the laser is used to generate a laser beam into the frequency-adjustable photoacoustic spectrometer to stimulate the photoacoustic effect of molecules in the gas to be measured; and the signal demodulator is electrically connected to the frequency-adjustable photoacoustic spectrometer to demodulate the electrical signal generated by the quartz tuning fork.

11. The gas detection device according to claim 10, characterized in that, It further includes a function generator, an adder, a laser driver, a laser, an optical fiber collimating lens, and a computer device; the modulation signal output end of the function generator is sequentially connected to the adder, the laser driver, and the laser; the synchronization signal output end of the function generator is connected to the signal demodulator; the computer device is further respectively connected to the output end of the signal demodulator and the input end of the function generator.

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