Tuning fork and gas detection device

Abstract

The invention relates to the technical field of gas sensing, and discloses a tuning fork and a gas detection device.The tuning fork comprises a crystal oscillator substrate, the tops of the two sides of the crystal oscillator substrate are each provided with a vibration arm extending upwards, a first gap is reserved between the two vibration arms, and arc notches are symmetrically formed in the opposite sides of the two vibration arms; the two arc notches and the first gap jointly form a circular light passing through hole penetrating through the tuning fork, and the tuning fork is provided with the circular light passing through hole, so that the size of a light path channel is remarkably increased, collimation of excitation light beams is facilitated, the effective mass of the tuning fork is reduced, the mass required to be driven by the vibration arm in the resonance process is lower, and the resonance efficiency is improved. Through comprehensive design of the diameter of the through hole and the thickness, the width and the length of the vibration arm, the resonant frequency of the tuning fork is improved, meanwhile, the high quality factor of the tuning fork is enhanced, the comprehensive noise reduction effect is remarkable, and the performance of the whole quartz enhanced photoacoustic spectrometry system is remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of gas sensing, and in particular to a tuning fork and a gas detection device. Background Art

[0002] Trace gas detection technology has been widely used in many fields, including industrial process control, medical diagnosis, environmental monitoring, etc. Traditional detection technologies, such as non-optical technologies such as gas chromatography / mass spectrometry and electrochemistry, have technical defects such as high cost, complex structure and slow reaction speed. Optical sensing technology has the characteristics of high sensitivity, strong selectivity and fast response speed and has been widely studied in recent years. Among the many optical sensing technologies, photoacoustic spectroscopy has unique advantages because it does not require the use of photodetectors. The principle of photoacoustic spectroscopy is based on the photoacoustic effect. After the target gas absorbs modulated light radiation, sound waves are generated due to changes in local pressure and temperature, and then an acoustic transducer such as a microphone or cantilever beam is used to convert the acoustic signal into an electrical signal. Compared with direct absorption spectroscopy, wavelength modulation spectroscopy, cavity enhanced spectroscopy and other methods, the biggest advantage of photoacoustic spectroscopy is that the strength of the photoacoustic effect does not depend on the length of the optical absorption path. Therefore, the size of the photoacoustic spectroscopy instrument can be miniaturized and modularized, and the structure is compact. Another advantage of photoacoustic spectroscopy is that the detection part of the photoacoustic spectroscopy detects acoustic wave signals instead of optical signals. Therefore, the photoacoustic spectroscopy instrument does not need to use photodetectors. The detection part of the photoacoustic spectroscopy is not limited by the optical wavelength of its excitation part. An acoustic wave transducer can be used to detect the acoustic wave signals generated by deep ultraviolet lasers and mid-infrared lasers. This advantage makes the application range of photoacoustic spectroscopy technology wider and reduces the cost of instruments based on photoacoustic spectroscopy technology.

[0003] As a variant of photoacoustic spectroscopy, quartz enhanced photoacoustic spectroscopy (QEPAS) has developed rapidly in recent years and has been used in aerospace, industrial process control, medical diagnosis and other fields. The principle of QEPAS is to use a tuning fork crystal made of quartz as an acoustic transducer, which accumulates acoustic energy through the piezoelectric effect. Using a tuning fork crystal instead of a traditional photoacoustic cell reduces the limitations of acoustic resonance conditions. The outstanding features of QEPAS are compact structure, low cost and strong noise resistance. The high noise resistance of QEPAS can be attributed to the narrow resonant response bandwidth caused by the high quality factor of the tuning fork crystal. In order to further enhance the signal of QEPAS, an acoustic microresonator is configured around the crystal to form an acoustic resonance. The acoustic microresonator made of a thin stainless steel tube is acoustically coupled to the tuning fork crystal to confine the acoustic wave and form a resonance to enhance the signal strength of QEPAS. By using a quartz enhanced photoacoustic spectroscopy sensor, Professor Spagnolo of the Polytechnic University of Bari, Italy, achieved 10-(12) level detection of sulfur hexafluoride SF6. Professor Ma Yufei of Harbin Institute of Technology demonstrated the use of tuning fork crystal oscillators as acoustic wave transducers in spatially distributed gas detection.

[0004] Public data shows that the shape of the tuning fork crystal oscillator currently in use is similar to the tuning fork instrument used in music. The overall shape is U-shaped, with two vibration arms arranged in parallel. The gap between the two vibration arms is about 0.25-0.35mm, and the gap between the vibration arms is narrow. This narrow gap places strict requirements on the beam quality of the excitation light source. If the light beam touches the vibration arm or the radiation coverage is uneven, background noise will be introduced due to local thermal effects or mechanical vibrations, significantly reducing the detection sensitivity. Therefore, a high-precision collimation optical system is required to ensure that the light beam passes through the gap "cleanly" to avoid contact with the vibration arm. However, the fundamental frequency absorption band of gas molecules is generally in the mid-infrared band. Currently, the main light sources that can produce this laser are quantum cascade lasers, etc. Quantum cascade lasers are limited by their working principles and internal structures, and the beam quality of this type of laser is poor. At the same time, affected by the diffraction limit, mid-infrared beam shaping has more technical difficulties than near-infrared and visible light. This design imposes strict requirements on the laser's beam collimation and the precision of the optical system, limiting the application of light sources with poor beam quality in this technology. In particular, when using a mid-infrared light source with a longer wavelength or an incoherent light source with poor coherence as an excitation light source, due to the influence of the light source beam quality, the light beam cannot "clean" through the narrow arm gap of the tuning fork crystal oscillator. Any contact or radiation of the light beam on the arm of the tuning fork crystal oscillator will generate a large background noise, resulting in a sharp drop in the detection sensitivity of quartz enhanced photoacoustic spectroscopy technology.

[0005] In addition, in photoacoustic spectroscopy technology, low-frequency noise (commonly referred to as 1 / f noise) has a particularly significant impact on the detection sensitivity and signal quality of the system. The sources of low-frequency noise are varied, including thermal noise, current fluctuations, mechanical vibrations, and environmental interference. Studies have found that the power density of low-frequency noise (commonly referred to as 1 / f noise) is correlated with the resonant frequency of the tuning fork. The current traditional tuning fork crystal oscillator has a resonant frequency of 32.7kHz at one atmosphere. At this resonant frequency, the power density of low-frequency noise is relatively high, resulting in a significant decrease in the ratio of signal power to noise power, that is, the signal-to-noise ratio (SNR), thereby affecting the reliability and sensitivity of the detector. Summary of the invention

[0006] The technical problem to be solved by the present invention is: the gap between the arms of a traditional tuning fork crystal oscillator is relatively narrow, and the light beam cannot "cleanly" pass through the narrow gap between the arms of the tuning fork crystal oscillator, thereby affecting the detection sensitivity; and the traditional tuning fork crystal oscillator has a resonant frequency of 32.7kHz at one atmosphere of pressure. At this resonant frequency, the power density of low-frequency noise is relatively high, resulting in a significant decrease in the signal-to-noise ratio (SNR), thereby affecting the reliability and sensitivity of the detector.

[0007] In order to solve the above technical problems, the present invention provides a tuning fork, a tuning fork, comprising a crystal oscillator base, two tops of the crystal oscillator base are respectively provided with a vibration arm extending upward, a first gap is left between the two vibration arms, and arc notches are symmetrically provided on the sides of the two vibration arms facing each other, and the two arc notches and the first gap constitute a circular light-passing through hole that penetrates the tuning fork;

[0008] Define the thickness of the tuning fork as φ, the diameter of the circular light-through hole as D, the width of the first gap as G, the distance between the top of the vibration arm and the top of the crystal oscillator base as I, the distance between the top of the vibration arm and the bottom of the crystal oscillator base as L, and the distance between the center of the circular light-through hole and the top of the crystal oscillator base as H;

[0009] The size of φ is 0.22mm to 0.38mm, the ratio of D / G is 2.24 to 2.52, the ratio of H / I is 0.78 to 0.84, the ratio of G / L is 0.03 to 0.07, and the ratio of I / L is 0.61 to 0.69.

[0010] Preferably, the size of φ is 0.25 mm to 0.35 mm, the ratio of D / G is 70:31 to 80:32, the ratio of H / I is 4:5 to 31:38, the ratio of G / L is 1:22 to 1:18, and the ratio of I / L is 19:30 to 2:3.

[0011] Preferably, φ=0.3 mm, L=6 mm, G=0.3 mm, D=0.7 mm, H=3.1 mm, and I=3.8 mm.

[0012] Preferably, the resonance frequency of the tuning fork is f, and the effective mass of the vibrating arm is m eff ,

[0013] The diameter of the circular light-through hole is D and the effective mass of the arm is m. eff The relationship is

[0014]

[0015] Thus, it is concluded

[0016] Preferably, the bottom of the crystal oscillator substrate is also provided with a pin extending downward.

[0017] Preferably, the material of the crystal oscillator substrate, each oscillator arm and each pin is quartz.

[0018] Preferably, the crystal oscillator base, each oscillator arm and each pin are integrally formed.

[0019] The present invention also provides a gas detection device, comprising the above-mentioned tuning fork, and also comprising a function generator, an adder, a laser driver and a laser;

[0020] The adder is connected to the modulation signal output end of the function generator, the laser driver is connected to the signal output end of the adder, the laser is driven by the laser driver, and the laser driver is used to control the injection current and temperature of the laser;

[0021] A lens group is arranged on the emission light path of the laser. The light beam emitted by the laser passes through the lens group and the circular light through hole in sequence. The lens group is used to focus the light beam emitted by the laser.

[0022] Preferably, it also includes a preamplifier and a lock-in amplifier;

[0023] The preamplifier is connected to each pin, and the lock-in amplifier is connected to the output end of the preamplifier;

[0024] The lock-in amplifier is connected to a synchronization signal output terminal of the function generator;

[0025] The preamplifier is used to collect and amplify the electrical signal of the tuning fork and transmit the electrical signal to the lock-in amplifier;

[0026] The lock-in amplifier is used to sequentially measure the output voltage of the tuning fork corresponding to sinusoidal excitation signals of different frequencies.

[0027] Preferably, it also includes a computer device, which has a data acquisition card, and the data acquisition card is respectively connected to the output end of the phase-locked amplifier and the input end of the function generator.

[0028] Compared with the prior art, the tuning fork and the gas detection device of the embodiment of the present invention have the following beneficial effects:

[0029] The embodiment of the present invention significantly increases the size of the optical path by arranging a circular light-through hole on the tuning fork, which is beneficial to the collimation of the excitation light beam and avoids the light beam from touching or radiating the vibration arm of the tuning fork crystal oscillator to generate a large background noise. The arrangement of the circular light-through hole also reduces the effective mass of the tuning fork, so that the mass required to drive the vibration arm during the resonance process is lower, thereby reducing the thermal noise caused by the molecular motion of the material itself. The circular light-through hole also optimizes the heat conduction path of the tuning fork, making the overall temperature gradient of the tuning fork more uniform, reducing the generation of local overheating or overcooling areas, thereby reducing the influence of temperature changes on the vibration stability of the tuning fork.

[0030] In addition, the embodiment of the present invention improves the resonance frequency of the tuning fork by comprehensively designing the diameter of the through hole, the thickness, width and length of the vibration arm, so that the working frequency of the tuning fork is far away from the main distribution range of the low-frequency noise, significantly reduces the 1 / f component of the low-frequency noise in the working of the tuning fork, and significantly improves the signal-to-noise ratio (SNR). Compared with the traditional 32.7kHz tuning fork, it has a higher signal-to-noise ratio (SNR) and detection sensitivity, thereby improving signal quality and sensitivity;

[0031] The embodiment of the present invention enhances the high quality factor (Q value) of the tuning fork by comprehensively designing the diameter of the through hole, the thickness, width and length of the vibration arm. The higher the Q factor, the narrower the frequency response bandwidth of the tuning fork, which means that the tuning fork can better filter external broadband noise (including thermal noise, mechanical noise and electrical noise), thereby further improving the signal-to-noise ratio of the photoacoustic spectroscopy detection system. In addition, the vibration of the tuning fork with a high Q factor decays slowly and can maintain high-amplitude vibration for a longer time, thereby enhancing the stability of signal detection and improving the detection capability of weak gas signals.

[0032] Under the synergistic effect of arranging a circular light-passing hole on the tuning fork to reduce the photothermal background noise caused by scattered light irradiating the tuning fork and the high resonant frequency tuning fork to reduce the 1 / f noise of the enhanced photoacoustic spectroscopy system, the tuning fork provided by the present invention has a significant comprehensive noise reduction effect, which significantly improves the performance of the entire quartz enhanced photoacoustic spectroscopy (QEPAS) system. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 is a three-dimensional model diagram of a tuning fork provided according to an embodiment of the present invention;

[0034] Figure 2 is a three-dimensional side view model diagram of a tuning fork provided according to an embodiment of the present invention;

[0035] Figure 3 is a plan view of a tuning fork provided according to an embodiment of the present invention;

[0036] Figure 4It is a vibration mode diagram of a tuning fork provided by an embodiment of the present invention simulated by COMSOL software;

[0037] Figure 5 is a frequency response curve diagram of a tuning fork provided according to an embodiment of the present invention;

[0038] Figure 6 is a schematic diagram of a tuning fork photoacoustic spectrum provided according to an embodiment of the present invention;

[0039] Figure 7 is a structural block diagram of a gas detection device provided according to an embodiment of the present invention;

[0040] Figure 8 is a noise comparison diagram of the tuning fork provided by the embodiment of the present invention and the ordinary tuning fork in the absence of light;

[0041] Fig. 9 is a schematic diagram of ammonia signals of different concentrations obtained by a detection device provided in an embodiment of the present invention;

[0042] Fig.10 It is a schematic diagram of a tuning fork combined with a coaxial micro-acoustic resonant cavity provided in an embodiment of the present invention.

[0043] In the figure, 1. computer equipment; 2. function generator; 3. adder; 4. laser driver; 5. laser; 6. lens group; 8. preamplifier; 9. phase-locked amplifier; 11. crystal oscillator base; 12. oscillator arm; 13. first gap; 14. circular light-through hole; 15. pin. DETAILED DESCRIPTION

[0044] The specific implementation of the present invention is further described in detail below in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0045] It should be noted that, unless otherwise specified, when a feature is referred to as being "fixed" or "connected" to another feature, it may be directly fixed or connected to the other feature, or it may be indirectly fixed or connected to the other feature. In addition, the descriptions of up, down, left, right, top, bottom, etc. used in the present invention are only relative to the relative positional relationship of the components of the present invention in the drawings.

[0046] In addition, unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art. The terms used in this specification are only for describing specific embodiments, not for limiting the present invention.

[0047] It should be understood that, although the terms first, second, third, etc. may be used to describe various elements in the present disclosure, these elements should not be limited to these terms. These terms are only used to distinguish elements of the same type from each other. For example, without departing from the scope of the present disclosure, the first element may also be referred to as the second element, and similarly, the second element may also be referred to as the first element.

[0048] Reference Figures 1 to 3 As shown, a preferred embodiment of the present invention provides a tuning fork, which is used for photoacoustic spectroscopy detection. The tuning fork includes a crystal oscillator substrate 11, and the tops of both sides of the crystal oscillator substrate 11 are respectively provided with an upwardly extending vibration arm 12, a first gap 13 is left between the two vibration arms 12, and arc notches are symmetrically provided on the opposite sides of the two vibration arms 12. The two arc notches and the first gap 13 together constitute a circular light-passing hole 14 that passes through the tuning fork, and the bottoms of both sides of the crystal oscillator substrate 11 are respectively provided with a downwardly extending pin 15. The material of the crystal oscillator substrate 11, each of the vibration arms 12 and each of the pins 15 is quartz, and the crystal oscillator substrate 11, each of the vibration arms 12 and each of the pins 15 are integrally formed.

[0049] Define the thickness of the tuning fork as φ, the diameter of the circular light-through hole 14 as D, the width of the first gap 13 as G, the distance between the top of the vibration arm 12 and the top of the crystal oscillator base 11 as I, the distance between the top of the vibration arm 12 and the bottom of the crystal oscillator base 11 as L, and the distance between the center of the circular light-through hole 14 and the top of the crystal oscillator base 11 as H, φ=0.3mm, L=6mm, G=0.3mm, D=0.7mm, H=3.1mm, I=3.8mm.

[0050] The vibration mode of the tuning fork was simulated by COMSOL software based on the finite element analysis method, and the results were obtained. Figure 4 According to the simulation results, the theoretical resonant frequency of the tuning fork obtained by finite element analysis is 35.6kHz.

[0051] Figure 6 The schematic diagram of the tuning fork photoacoustic spectroscopy technology is shown. The light beam is focused through the circular light through hole 14 of the tuning fork, and the light interacts with the target molecule to generate sound waves, which drive the vibrating arm 12 of the tuning fork to vibrate symmetrically. When the tuning fork vibrates, an electrical signal is generated due to the piezoelectric effect. The electrical signal is exported through the pin 15 of the tuning fork for signal processing. The size of the electrical signal is positively correlated with the concentration of the target molecule being measured, thereby achieving the purpose of gas detection.

[0052] The resonant frequency of a quartz tuning fork is determined by its geometric dimensions and material properties. According to the Euler-Bernoulli beam theory, for a cantilever beam with one end fixed and the other end free, the resonant frequency formula of its basic vibration mode is:

[0053]

[0054] The moment of inertia I of the boom cross section, where the boom width w and thickness t is:

[0055]

[0056] According to the quartz density ρ, the cross-sectional area A of the vibration arm, and the effective length L of the vibration arm, the mass m and stiffness k of the vibration arm can be calculated:

[0057] m=ρAL=ρwtL

[0058]

[0059] Among the material properties, Young's modulus E and density ρ of quartz determine the elasticity and mass distribution of the material. Common material parameters are as follows:

[0060] E=72GPa

[0061] ρ=2650kg / m 3

[0062] Substituting the above formula and material properties into the resonant frequency formula, we get:

[0063]

[0064] The present invention changes the geometric shape of the tuning fork and processes a circular light-passing hole 14 with a diameter D on the tuning fork, thereby changing the effective mass m of the vibration arm. eff , we can get the diameter D of the circular light-passing hole 14 and the effective mass m of the vibrating arm eff Relationship:

[0065]

[0066] Go there,

[0067] The tuning fork resonant frequency f is obtained as:

[0068]

[0069] It can be seen that the increase in the diameter D of the circular through hole 14 will lead to an increase in the effective mass m eff Reduced, because the existence of the circular light-passing hole 14 reduces the effective volume of the vibration arm 12, further deduction shows:

[0070] When D increases reduce;

[0071] And Δf and Directly proportional.

[0072] Therefore, we can get:

[0073]

[0074] It can be obtained from the formula that through this relationship, the resonant frequency f of the tuning fork can be effectively improved by reasonably designing the diameter D of the circular light-passing hole 14 and the geometric parameters of the vibration arm 12. The present invention not only improves the resonant frequency of the tuning fork, but also optimizes the anti-noise ability and detection sensitivity of the system by appropriately adjusting the diameter D of the circular light-passing hole 14 and the geometric dimensions of the vibration arm 12.

[0075] It can be seen from the above formula that the present invention effectively reduces the mass of the vibration arm 12 through the design of the circular light-passing hole 14, thereby improving the resonant frequency of the tuning fork. At the same time, the reduction in effective mass helps to improve the Q factor of the tuning fork, reduce the ineffective vibration loss of the tuning fork, and improve signal stability.

[0076] In addition, the traditional tuning fork design is easily affected by external temperature fluctuations due to the closed arm structure, large material volume, and high heat capacity, resulting in significant thermal noise. In the photoacoustic spectroscopy detection system, the photothermal noise mainly comes from the laser irradiating the tuning fork arm, and part of the light is absorbed by the arm material and converted into heat energy, resulting in a local temperature increase, thereby causing thermal expansion and structural stress changes in the tuning fork. This temperature change will interfere with the resonant characteristics of the tuning fork, resulting in additional low-frequency noise. The circular through-light hole 14 of the present invention also reduces the photothermal noise caused by the light beam irradiating the arm, and can also reduce the thermal expansion and contraction effect caused by the intrinsic thermal noise of the material and the ambient temperature change in the absence of light, so that the tuning fork can maintain a higher signal-to-noise ratio and detection sensitivity under different working conditions. Thermal noise is one of the key factors affecting the stability and detection limit of the photoacoustic spectroscopy detection system. Its sources include the thermoelastic noise inside the material, the thermal expansion effect caused by ambient temperature fluctuations, and the local temperature change caused by laser irradiation of the tuning fork arm.

[0077] The through-hole structure adopted in the present invention reduces the effective mass of the tuning fork, so that the mass required to drive the vibration arm 12 during the resonance process is lower, thereby reducing the thermal noise caused by the molecular movement of the material itself. In addition, the through-hole structure optimizes the heat conduction path of the tuning fork, making the overall temperature gradient of the tuning fork more uniform, reducing the generation of local overheating or overcooling areas, thereby reducing the impact of temperature changes on the vibration stability of the tuning fork.

[0078] The through-hole structure used in the present invention effectively reduces the probability of the light beam irradiating the vibrating arm, so that the laser beam can "clean" the through-hole gap of the tuning fork, avoiding the scattering and absorption effect of the light beam on the surface of the tuning fork, thereby further reducing the generation of photothermal noise. In addition, since the tuning fork works in high-frequency mode (35.6kHz), compared with the traditional 32kHz tuning fork, the 1 / f noise of its signal is greatly reduced, further improving the signal-to-noise ratio of the photoacoustic spectroscopy system.

[0079] In the absence of light, the tuning fork provided by the present invention can still show lower thermal noise. This is because the through-hole design reduces the overall material volume of the tuning fork, which reduces its heat capacity, thereby reducing the thermal noise caused by temperature fluctuations. In addition, the tuning fork of the traditional closed structure is prone to produce a large thermal expansion and contraction effect when the ambient temperature changes, affecting the mechanical vibration stability of the tuning fork, while the tuning fork with a through-hole structure can effectively reduce the dimensional changes caused by ambient temperature fluctuations due to the reduced material and more uniform heat conduction path, thereby improving the vibration stability of the tuning fork under different temperature conditions.

[0080] High Q factor is also crucial for reducing thermal noise. In the design of the present invention, by optimizing the geometric structure of the tuning fork vibration arm (such as shortening the vibration arm length, increasing the vibration arm thickness, optimizing the cross-sectional area, etc.), not only the resonant frequency of the tuning fork is effectively improved, but also its Q factor is greatly improved. The higher the Q factor, the narrower the frequency response bandwidth of the tuning fork, which means that the tuning fork can better filter external broadband noise, including thermal noise, mechanical noise and electrical noise, thereby further improving the signal-to-noise ratio of the photoacoustic spectroscopy detection system. In addition, the tuning fork vibration of the high Q factor decays slowly, and high amplitude vibration can be maintained for a longer time, thereby enhancing the stability of signal detection and improving the detection capability of weak gas signals.

[0081] In order to experimentally verify the resonant frequency of the tuning fork, the present invention uses a function generator and a phase-locked amplifier to measure the frequency response curve of the tuning fork. The function generator generates a series of sinusoidal excitation signals, the amplitude of the sinusoidal excitation signal is fixed at 400mV, and the frequency is scanned from 35635Hz to 35685Hz with a step size of 0.1Hz. The phase-locked amplifier is used to measure the output voltage of the tuning fork corresponding to the sinusoidal excitation signal of different frequencies in turn, and the following is obtained: Figure 5 The frequency response curve shown in Figure 1 is shown in Figure 1. Using Lorenz linear fitting, the center frequency f of the frequency response curve is obtained. 0 is 35660Hz, and the half-width Δf is 3.37Hz. According to Q=f 0 / Δf gives the quality factor Q of the tuning fork as 10582.

[0082] Therefore, the following conclusions can be drawn from the above Experiment 2: The Q factor of the tuning fork is about 10 of that of the conventional photoacoustic spectroscopy photoacoustic cell. 2 times, which is conducive to the accumulation of photoacoustic energy and helps to improve the detection sensitivity of photoacoustic spectroscopy.

[0083] In other embodiments, the dimension of φ is 0.22 mm to 0.38 mm, the ratio of D / G is 2.24 to 2.52, the ratio of H / I is 0.78 to 0.84, the ratio of G / L is 0.03 to 0.07, and the ratio of I / L is 0.61 to 0.69.

[0084] In other embodiments, the size of φ is 0.25 mm to 0.35 mm, the ratio of D / G is 70:31 to 80:32, the ratio of H / I is 4:5 to 31:38, the ratio of G / L is 1:22 to 1:18, and the ratio of I / L is 19:30 to 2:3.

[0085] In other embodiments, the crystal oscillator substrate 11 , each oscillator arm 12 , and each pin 15 are all made of aluminum.

[0086] In other embodiments, the crystal oscillator substrate 11 , each oscillator arm 12 , and each pin 15 are all made of steel.

[0087] In other embodiments, the crystal oscillator substrate 11 , the oscillator arms 12 and the pins 15 may also be provided separately.

[0088] Reference Figure 6 Combination Figure 7 As shown, in some embodiments, the present invention discloses a gas detection device, comprising a tuning fork according to the above structure.

[0089] The specific working process of the gas detection device of the present invention is as follows:

[0090] The specific connection structure of the gas detection device is as follows: function generator 2; adder 3 connected to the modulation signal output end of function generator 2; laser driver 4 connected to the signal output end of adder 3; laser 5 driven by laser driver 4, laser 5 is used to emit at least mid-infrared light; lens group 6 arranged in the emission light path of the laser 5; the tuning fork arranged in the emission light path of lens group 6; preamplifier 8 connected to pin 15 of the tuning fork; phase-locked amplifier 9 connected to the output end of preamplifier 8, phase-locked amplifier 9 is connected to the synchronization signal output end of function generator 2. Specifically, a computer device 1 having a data acquisition card, the data acquisition card is connected to the output end of phase-locked amplifier 9 and the input end of function generator 2 respectively.

[0091] The above-mentioned gas detection device is used to set up the experimental environment as follows: the gas detection device is set in a closed space, and ammonia water is added by a pipette to obtain ammonia gas through volatilization. The pipette can add uL10 each time. -6 L-level ammonia water is used to obtain trace concentrations of ammonia gas. The concentration of ammonia gas produced in a confined space is positively correlated with the volume of ammonia water added, and the specific ammonia concentration can be calculated in detail using the formula.

[0092] The working principle of the gas detection device is as follows: the computer device 1 uses the Labview program to control the function generator 2. The device uses a dual-channel function generator. The function generator 2 generates a frequency of f 0 The sinusoidal signal, f 0It is the resonant frequency of the tuning fork and a DC bias signal. The two signals are added by the adder 3, and the added signal is transmitted to the laser driver 4, which can control the injection current and temperature of the laser 5. The output of the laser 5 is amplitude modulated. The light beam emitted by the laser 5 is focused by the lens group 6 and passes through the circular light-through hole 14 of the tuning fork. The electrical signal generated by the tuning fork is amplified by the preamplifier 8, which uses a transimpedance preamplifier, and then transmitted to the phase-locked amplifier 9. The reference signal demodulated by the phase-locked amplifier comes from the synchronization port of the function generator 2. The signal is transmitted back to the computer device 1 for display after being demodulated in the 1f mode.

[0093] In specific application, the device of the present invention can display the measured gas concentration on a computer in real time and online. The device has the functions of high precision, strong compactness, and online monitoring.

[0094] Among them, the laser 5 uses a mid-infrared quantum cascade laser with a wavelength of 9.7μm as the excitation light source, and the light source in the detection device adopts amplitude modulation. The control electronic unit is used as a laser driver to control the laser chip temperature and injection current, and the laser chip is controlled at 25°C. Function generator 2 is a dual-channel function generator, which generates a 650mA DC bias signal and a frequency f 0 =35360Hz sine signal, where f 0 is the resonant frequency of the tuning fork. The laser 5 uses a lens with a focal length of 30 mm to focus the laser beam through the circular light through hole 14 of the tuning fork. Figure 3 The centers of the through-hole-shaped recesses 14 shown in FIG. 1 coincide with each other.

[0095] During the experiment, when the photoacoustic effect occurs, light acts on ammonia molecules to excite the molecules from the ground state to a high-energy state. The high-energy state molecules generate local heat due to collision deexcitation, causing the surrounding air medium to expand and contract, thereby generating sound waves. Sound waves are generated around the light beam and are coaxially distributed in a columnar shape with the light beam. The sound wave pressure drives the vibrating arm 12 of the tuning fork to vibrate. The tuning fork generates an electric charge through the piezoelectric effect. The electrical signal is first amplified by the preamplifier 8 with a feedback resistance of 10MΩ, and then transmitted to the phase-locked amplifier for phase-locked amplification. The phase-locked amplifier demodulates the signal in the 1f mode to obtain an electrical signal related to the concentration of gas molecules. The control system of the detection device is controlled by a self-written LabView program, and the ammonia concentration calculated based on the 1f signal is displayed by the LabView program.

[0096] Before the measurement, the noise suppression performance of the tuning fork was evaluated. The experimental conditions were: turning off the laser 5, not adding ammonia water, and measuring at room temperature and pressure. The noise floor of the tuning fork with a frequency of 35.6kHz described in the present invention was compared with the noise floor of the standard tuning fork with a frequency of 32.7kHz. The experimental results are shown in Figure 8 .

[0097] The results show that the standard deviation of the tuning fork's noise is only 1.7μV, which is 50% lower than that of the standard tuning fork, indicating that it has better performance in suppressing 1 / f noise, thermal noise, and airflow noise.

[0098] After adding ammonia water, turning on the laser light source, and conducting a detection experiment, the relationship between the amplitude of the ammonia photoacoustic signal and the volume of ammonia water obtained in the experiment is as follows: Fig. 9 The experimental results verify the qualitative relationship between the photoacoustic signal obtained by the gas detection device and ammonia, and the experimental results verify the ammonia detection capability of the device.

[0099] Therefore, the following conclusions were drawn from the above experiments: the detection device showed that the amplitude of the photoacoustic signal was positively correlated with the volume of ammonia water, and the linear correlation coefficient was 0.95, demonstrating that the detection device had the ability to detect ammonia molecules.

[0100] In summary, the tuning fork has a newly designed structure and shape, and after geometric parameter design, it has a high-frequency resonance frequency of 35.6kHz, which is suitable for photoacoustic spectroscopy detection; the vibrating arm of the tuning fork is in the shape of a through hole, and a unique through-hole-shaped gap space is designed between the vibrating arms, which can be beneficial to the collimation of the excitation light beam. In addition, the through-hole design of the gap space is consistent with the wavefront shape of the cylindrical sound wave, which can collect sound waves more efficiently and improve the efficiency of acoustic-electric energy conversion. The present invention applies the tuning fork photoacoustic spectroscopy technology to a gas detection device. When testing with ammonia, the device can clearly distinguish ammonia of different concentrations and show a linear response of more than 95%; the photoacoustic spectroscopy detection device based on the tuning fork has the characteristics of small sampling volume and fast response speed. By changing the laser wavelength, it can realize the detection of many other gases, which can effectively improve the detection ability of the photoacoustic spectrum.

[0101] Fig.10It is a schematic diagram of the assembly of the high-frequency through-hole tuning fork and the coaxial acoustic resonant cavity of the present invention. In order to make the coaxial acoustic resonant cavity 16 achieve acoustic field resonance, its optimal cavity length is about half of the wavelength of the sound wave. According to the calculation that the speed of sound is about 340m / s at normal temperature and pressure, when a standard tuning fork with a frequency of about 32.7kHz is used, the total cavity length of the acoustic resonant cavity is about 10.4mm; and when the tuning fork provided by the present invention is used, the total cavity length of the acoustic resonant cavity is only about 9.5mm. Taking into account that the excitation light beam has a certain divergence angle, the light beam will generate great background noise when scattered to any position of the tuning fork and the cavity. When the tuning fork provided by the present invention is used in conjunction with the resonant cavity, a better light beam collimation effect can be achieved, thereby effectively reducing the scattered light background noise, showing obvious advantages.

[0102] In summary, the embodiments of the present invention provide a tuning fork and a gas detection device. The embodiments of the present invention significantly increase the size of the optical path by arranging a circular light-through hole on the tuning fork, which is beneficial to the collimation of the excitation light beam and avoids the light beam from touching or radiating the vibration arm of the tuning fork crystal oscillator to generate a large background noise. The arrangement of the circular light-through hole also reduces the effective mass of the tuning fork, so that the mass required to drive the vibration arm during the resonance process is lower, thereby reducing the thermal noise caused by the molecular motion of the material itself. The circular light-through hole also optimizes the heat conduction path of the tuning fork, so that the overall temperature gradient of the tuning fork is more uniform, reducing the generation of local overheating or overcooling areas, thereby reducing the influence of temperature changes on the vibration stability of the tuning fork.

[0103] In addition, the embodiment of the present invention improves the resonance frequency of the tuning fork by comprehensively designing the diameter of the through hole, the thickness, width and length of the vibration arm, so that the working frequency of the tuning fork is far away from the main distribution range of the low-frequency noise, significantly reduces the 1 / f component of the low-frequency noise in the working of the tuning fork, and significantly improves the signal-to-noise ratio (SNR). Compared with the traditional 32.7kHz tuning fork, it has a higher signal-to-noise ratio (SNR) and detection sensitivity, thereby improving signal quality and sensitivity;

[0104] The embodiment of the present invention enhances the high quality factor (Q value) of the tuning fork by comprehensively designing the diameter of the through hole, the thickness, width and length of the vibration arm. The higher the Q factor, the narrower the frequency response bandwidth of the tuning fork, which means that the tuning fork can better filter external broadband noise (including thermal noise, mechanical noise and electrical noise), thereby further improving the signal-to-noise ratio of the photoacoustic spectroscopy detection system. In addition, the vibration of the tuning fork with a high Q factor decays slowly and can maintain high-amplitude vibration for a longer time, thereby enhancing the stability of signal detection and improving the detection capability of weak gas signals.

[0105] Under the synergistic effect of arranging a circular light-passing hole on the tuning fork to reduce the photothermal background noise caused by scattered light irradiating the tuning fork and the high resonant frequency tuning fork to reduce the 1 / f noise of the enhanced photoacoustic spectroscopy system, the tuning fork provided by the present invention has a significant comprehensive noise reduction effect, which significantly improves the performance of the entire quartz enhanced photoacoustic spectroscopy (QEPAS) system.

[0106] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and substitutions can be made without departing from the technical principles of the present invention. These improvements and substitutions should also be regarded as the scope of protection of the present invention.

Claims

1. A tuning fork, comprising a crystal oscillator base (11), wherein the tops of both sides of the crystal oscillator base (11) are respectively provided with a vibration arm (12) extending upward, and a first gap (13) is left between the two vibration arms (12), characterized in that: Circular arc notches are symmetrically arranged on one side of the two vibration arms (12) facing each other, and the two circular arc notches and the first gap (13) form a circular light-passing through hole (14) that passes through the tuning fork; The thickness of the tuning fork is defined as φ, the diameter of the circular light-passing hole (14) is defined as D, the width of the first gap (13) is defined as G, the distance between the top of the vibration arm (12) and the top of the crystal oscillator base (11) is defined as I, the distance between the top of the vibration arm (12) and the bottom of the crystal oscillator base (11) is defined as L, and the distance between the center of the circular light-passing hole (14) and the top of the crystal oscillator base (11) is defined as H; The size of φ is 0.22mm to 0.38mm, the ratio of D / G is 2.24 to 2.52, the ratio of H / I is 0.78 to 0.84, the ratio of G / L is 0.03 to 0.07, and the ratio of I / L is 0.61 to 0.

69.

2. A tuning fork according to claim 1, characterized in that: The size of φ is 0.25mm to 0.35mm, the ratio of D / G is 70:31 to 80:32, the ratio of H / I is 4:5 to 31:38, the ratio of G / L is 1:22 to 1:18, and the ratio of I / L is 19:30 to 2:

3.

3. A tuning fork according to claim 2, characterized in that: The φ=0.3 mm, the L=6 mm, the G=0.3 mm, the D=0.7 mm, the H=3.1 mm, and the I=3.8 mm.

4. A tuning fork according to any one of claims 1 to 3, characterized in that: Assume the resonant frequency of the tuning fork is f and the effective mass of the vibrating arm is m eff , Then the diameter of the circular light-through hole (14) is D and the effective mass m of the vibration arm (12) is eff The relationship is Thus, it is concluded 5. A tuning fork according to any one of claims 1 to 3, characterized in that: The bottom of each crystal oscillator substrate (11) is also provided with a pin (15) extending downwards.

6. A tuning fork according to any one of claims 1 to 3, characterized in that: The material of the crystal oscillator base (11), each of the oscillator arms (12) and each of the pins (15) is quartz.

7. A tuning fork according to any one of claims 1 to 3, characterized in that: The crystal oscillator base (11), each of the oscillator arms (12) and each of the pins (15) are integrally formed.

8. A gas detection device, characterized in that: A tuning fork according to any one of claims 1 to 7, further comprising a function generator (2), an adder (3), a laser driver (4) and a laser (5); The adder (3) is connected to the modulation signal output end of the function generator (2), the laser driver (4) is connected to the signal output end of the adder (3), the laser (5) is driven by the laser driver (4), and the laser driver (4) is used to control the injection current and temperature of the laser (5); A lens group (6) is provided on the emission light path of the laser (5); the light beam emitted by the laser (5) passes through the lens group (6) and the circular light through hole (14) in sequence; the lens group (6) is used to focus the light beam emitted by the laser (5).

9. A gas detection device according to claim 8, characterized in that: It also includes a preamplifier (8) and a lock-in amplifier (9); The preamplifier (8) is connected to each of the pins (15), and the lock-in amplifier (9) is connected to the output end of the preamplifier (8); The phase-locked amplifier (9) is connected to the synchronization signal output terminal of the function generator (2); The preamplifier (8) is used to collect and amplify the electrical signal of the tuning fork, and transmit the electrical signal to the lock-in amplifier (9); The lock-in amplifier (9) is used to sequentially measure the output voltages of the tuning fork corresponding to sinusoidal excitation signals of different frequencies.

10. A gas detection device according to claim 9, characterized in that: It also comprises a computer device (1), wherein the computer device (1) has a data acquisition card, and the data acquisition card is respectively connected to the output end of the phase-locked amplifier (9) and the input end of the function generator (2).

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