High-precision differential resonance photoacoustic cell based on multi-order acoustic mode and gas detection device

Abstract

The invention provides a high-precision differential resonance photoacoustic cell based on a multi-order acoustic mode and a gas detection device, and solves the problems that a mainstream dual-channel differential resonance photoacoustic cell only utilizes a fundamental mode, the acoustic mode utilization rate is low, a detected high-order mode acoustic signal is weak, and the photoacoustic cell cannot measure multi-component gas at high precision at the same time. According to the photoacoustic cell, the number of the microphones is matched with the multi-order acoustic mode, the microphones are mounted at the sound pressure extreme points in the resonant cavity under the corresponding mode, so that sound pressure signals are extracted to the maximum extent, and the photoacoustic cell has the functions of high sensitivity, high precision, high anti-interference capability and simultaneous detection of various gases; therefore, the gas detection device related to the photoacoustic cell comprises the differential amplification circuit, the circuit is matched with the photoacoustic cell, the utilization rate of an acoustic mode is improved, multiple gases can be measured at the same time, the detection precision is improved, the device is simple, and the detection cost is further reduced.

Description

Technical Field

[0001] The present invention belongs to the field of gas detection, and particularly relates to a high-precision differential resonant photoacoustic cell and a gas detection device based on multi-order acoustic modes. Background Art

[0002] Photoacoustic spectroscopy is an indirect absorption spectroscopy technology based on the photoacoustic effect. When a gas absorbs periodically modulated light energy, it transitions from the ground state to the excited state. When the molecule returns to the ground state, it converts the light energy into heat energy in the form of heat, resulting in a periodic change in the temperature of the substance. Due to thermal expansion and contraction, this temperature change causes a periodic change in the volume of the substance, thereby generating a pressure wave, i.e., a sound wave. By detecting the sound wave signal with a sound sensor and converting it into a voltage signal within the characteristic absorption wavelength range of the target gas, since the voltage signal is proportional to the gas concentration information, the concentration information of the target gas can be calculated. This technology does not consume the measured gas and can detect multiple gases by selecting lasers of different wavelengths. It has the advantages of zero background, no wavelength selectivity, detection sensitivity proportional to the excitation light power, small size, fast response, low cost, etc., and is widely used in fields such as environmental monitoring, medical diagnosis, and industrial applications. The sound sensor is one of the core components in photoacoustic spectroscopy gas sensing technology, and the photoacoustic cell is a commonly used sound sensor in this technology.

[0003] Currently, the mainstream differential resonant photoacoustic cell mainly consists of two resonant cavities with completely symmetric geometric structures, one of which is a reference cavity and the other is a signal cavity. Two microphones are respectively located at the longitudinal center positions of the two resonant cavities, i.e., at the positions where the sound signal of the resonant cavity is the strongest in the fundamental frequency mode. When the modulated laser irradiates the signal cavity containing the target gas, it will cause periodic thermal expansion due to gas absorption, thereby exciting sound waves. By performing differential amplification processing on the sound signals in the two cavities, background noise and environmental disturbances can be effectively suppressed, thereby improving the signal-to-noise ratio and stability of the detection system.

[0004] In the multi-gas synchronous detection device and method disclosed in the prior art CN119779992A, although the fundamental mode and high-order modes of the photoacoustic cell are utilized to achieve the simultaneous detection of multiple gases, there are significant limitations in terms of detection accuracy and the utilization of multi-order acoustic modes. This scheme only uses two microphones located at the longitudinal center position of the resonant cavity to detect the fundamental mode and high-order mode signals of the photoacoustic cell. However, since the acoustic signal intensity generated by the high-order modes is usually much lower than that of the fundamental mode, the detected high-order mode acoustic signals are weak, and the strength of the acoustic signal is related to the accuracy of gas detection, which affects the detection sensitivity and accuracy of the system for the corresponding gas. In addition, for some high-order acoustic modes, the acoustic pressure nodes at the longitudinal center of the resonant cavity are exactly zero, resulting in the microphones located at the center position of the resonant cavity being unable to effectively sense the photoacoustic signals in this mode, thus restricting the full utilization of multi-order acoustic modes and further restricting the overall performance of the system and the multi-gas detection ability.

[0005] The prior art also provides a scheme for detecting multiple gases by using multiple photoacoustic resonators. However, this scheme will inevitably cause a significant increase in the volume of the system, reducing the integration degree of the system. In addition, the increase in volume will make the diffusion and exchange process of gases relatively slow, increasing the response time of the system, which is not suitable for real-time gas detection scenarios that require rapid response. Summary of the Invention

[0006] To solve the above problems existing in the prior art, the present invention provides a high-precision differential resonant photoacoustic cell and gas detection device based on multi-order acoustic modes. The technical problems to be solved by the present invention are achieved through the following technical solutions:

[0007] In the first aspect, the present invention provides a high-precision differential resonant photoacoustic cell based on multi-order acoustic modes, including two resonant cavities and two buffer cavities. The two resonant cavities are arranged in parallel up and down. One buffer cavity is provided at each end of the two resonant cavities. On the side of each buffer cavity far from the resonant cavity, an optical window is installed for sealing. An air path inlet and outlet pipe is provided on each buffer cavity. A plurality of microphones are arranged outside each resonant cavity, and the plurality of microphones are arranged at the acoustic pressure extreme value points corresponding to the modes in the resonant cavity.

[0008] In the second aspect, the present invention provides a gas detection device based on multi-order acoustic modes, including: a signal generation and processing device and the differential resonant photoacoustic cell based on multi-order acoustic modes described in the first aspect. The differential resonant photoacoustic cell based on multi-order acoustic modes is used to detect the acoustic signals of the gas to be measured in the two resonant cavities; the signal generation and processing device is used to detect the concentration of the gas according to the acoustic signals of the gas.

[0009] Advantageous Effects:

[0010] 1. The high-precision differential resonance photoacoustic cell based on multi-order acoustic modes provided by the present invention improves the utilization rate of the acoustic modes of the photoacoustic cell by matching the number of microphones with the multi-order modes.

[0011] 2. In the existing dual-channel differential photoacoustic cell, usually only two microphones are used, and the strength of the detected sound pressure signal is related to the accuracy of gas detection. The present invention realizes the multi-fold amplification of the sound pressure signal through a simple and reasonable microphone arrangement scheme in cooperation with a differential amplification circuit, thereby improving the detection accuracy.

[0012] 3. The present invention takes advantage of the zero background, no wavelength selectivity, simple structure, small volume and low cost of the photoacoustic spectroscopy technology itself to improve the acoustic sensor in the photoacoustic spectroscopy technology. It not only improves the utilization rate of the acoustic modes, realizes the simultaneous measurement of multiple gases, but also realizes the amplification of the sound pressure signal, improves the system accuracy, and the device is simple, further reducing the cost;

[0013] 4. On the premise of keeping the acoustic structure and geometric parameters of the photoacoustic cell unchanged, the present invention can realize the enhanced acquisition of weak sound signals that are difficult to detect by two microphones originally. Compared with the traditional dual-microphone structure, this method significantly improves the detection sensitivity of the system without changing the structure of the photoacoustic cell, reduces the minimum detectable gas concentration (detection limit), and is especially suitable for high-precision detection scenarios of trace gases.

[0014] The present invention will be further described in detail below with reference to the drawings and embodiments. Brief Description of the Drawings

[0015] Figure 1 is a schematic diagram of a high-precision differential resonance photoacoustic cell based on multi-order acoustic modes that can simultaneously measure two gases provided by the present invention;

[0016] Figure 2 is a schematic diagram of a gas detection device based on multi-order acoustic modes provided by the present invention;

[0017] Figure 3 is the acoustic mode simulation cloud map provided by the present invention;

[0018] Figure 4 is the sound signal detected by the differential amplification circuit when only two microphones are used in the photoacoustic cell under the same conditions provided by the present invention;

[0019] Figure 5 is a schematic diagram of the frequency response curve graph after differential amplification using six microphones provided by the present invention;

[0020] Figure 6 is a schematic diagram of a high-precision differential resonance photoacoustic cell based on multi-order acoustic modes that can simultaneously measure three gases provided by the present invention. Detailed Embodiments

[0021] The present invention will be further described in detail below in conjunction with specific embodiments, but the implementation manners of the present invention are not limited thereto.

[0022] Referring to Figure 1 shown in the figure, the present invention provides a high-precision differential resonant photoacoustic cell based on multi-order acoustic modes, which includes two resonant cavities and two buffer cavities. The two resonant cavities are arranged in parallel up and down. One buffer cavity is arranged at each end of the two resonant cavities. An optical window is installed on one side of each buffer cavity away from the resonant cavity for sealing. An air passage inlet and outlet pipe is provided on each buffer cavity, and a plurality of microphones are arranged outside each resonant cavity. The microphones are installed in installation grooves, and the installation grooves are uniformly arranged on the top of the resonant cavity. The plurality of microphones are arranged at the sound pressure extreme points corresponding to the modes in the resonant cavity.

[0023] Referring to Figure 1 , in Figure 1 , 1 and 2 are resonant cavities, 3 and 4 are buffer cavities, 5 and 6 are optical windows, 7 and 8 are air passage inlet and outlet pipes, and 9, 10, 11, 12, 13, and 14 are microphones. The buffer cavities are hollow, the openings at both ends of the two resonant cavities are open, and the gas in the resonant cavities enters the buffer cavities through the open openings and circulates with each other through the buffer cavities.

[0024] To enhance the applicability and scalability of the present invention, the microphones are not limited to a specific model or type, and acoustic sensors including but not limited to electret microphones, MEMS microphones, capacitive microphones, etc. can be selected; the differential amplification circuit can be implemented by an analog differential amplification structure or a differential operation module based on digital signal processing; the cavity structure can be made of materials with good acoustic properties (such as aluminum metal, stainless steel, etc.); the window panes are made of materials with good optical transmission performance (such as quartz glass, calcium fluoride, etc.). The present invention is not limited to the above specific implementation materials and components.

[0025] The high-precision differential resonant photoacoustic cell based on multi-order acoustic modes of the present invention has two modes, namely multi-order in-phase modes and multi-order anti-phase modes. The high-precision differential resonant photoacoustic cell based on multi-order acoustic modes operates in the multi-order anti-phase mode. The installation quantity of the microphones on each resonant cavity is related to the order of the multi-order anti-phase mode, and the installation position and quantity of the microphones are determined by the position and quantity of the sound pressure extreme values.

[0026] The two modes of the photoacoustic cell provided by the present invention, namely the high-precision differential resonant photoacoustic cell based on multi-order acoustic modes, hereinafter referred to as the photoacoustic cell or the dual-channel differential photoacoustic cell, will be described in detail below.

[0027] The target gas in the photoacoustic cell generates a thermal wave due to the absorption of the modulated laser This thermal wave will excite an acoustic wave in the photoacoustic cell. Approximating the gas in the photoacoustic cell as an ideal gas, its wave equation can be expressed as:

[0028]

[0029] In the formula, p is the sound pressure, is the displacement vector, c is the sound speed of the gas in the photoacoustic cell, γ is the gas heat capacity ratio, is the thermal power density function, is the Laplace operator. When the light source is modulated as a sine wave, the change in the pressure p in the photoacoustic cell can be expanded in the superposition form of the normal mode p j :

[0030]

[0031] In the formula, is the acoustic normal mode, which is determined by the geometric structure of the photoacoustic cell and represents the standing wave form existing in the photoacoustic cell. For a single-channel photoacoustic cell, with a length of L c , the resonance frequency of the resonance photoacoustic cell with a radius of R in the normal mode can be expressed as:

[0032]

[0033] In the formula, the resonance frequency is related to the length and radius of the photoacoustic cell, and α jm is the m-th root of the j-th Bessel function, and j, m, and q are the orders of the radial, angular, and longitudinal modes of the normal mode respectively, which represent the sound field distribution in the photoacoustic cell. It can be seen from the above that there are three types of acoustic normal modes: angular, radial, and longitudinal. Since the angular and radial modes are difficult to excite and the sound pressure signals are difficult to detect, existing photoacoustic cells mostly only use the longitudinal mode, that is, α jm =α 00 =0.

[0034] For the high-precision differential resonance photoacoustic cell based on multi-order acoustic modes provided by the present invention, it can be seen from the above that the acoustic normal mode represents the standing wave form existing in the sound cell. For the longitudinal mode, to form a standing wave, the cavity must satisfy the resonance condition, L is the cavity length, and a is an integer. The present invention can explain the cavity length from two aspects: the total length (the length of two buffer cavities plus the resonance cavity) and the length of a single resonance cavity.

[0035] 1. In the case where the cavity length is the total length, since the length must be a half-wave number Integer multiples, the excitation source is only excited in one-sided resonant cavities. When the sound wave reaches the buffer cavity wall during propagation, it must be at the peak or trough of the wave. Then, reflected by the buffer cavity wall, the sound wave will pass through the other resonant cavity in the same form. Thus, it can be obtained that the sound waves in the two resonant cavities are exactly the same and there is no phase difference. We define this as the in-phase mode, which is represented by the letter "Bk", where the number k represents the number of half-waves contained in the total length of the photoacoustic cell (the length of two buffer cavities plus one resonant cavity). For example, "B2" means that two half-wavelengths are formed on the total length of the photoacoustic cell, that is, the in-phase acoustic mode forming a complete standing wave.

[0036] 2. When the cavity length is equal to the length of the resonant cavity, in this case, the sound wave does not reach the buffer cavity wall during propagation, but loops back at the end of the resonant cavity and then directly enters the other resonant cavity. The sound wave is only generated in the two resonant cavities and is a loop. The standing wave form is only related to the resonant cavity and has nothing to do with the buffer cavity. Thus, it can be obtained that the sound waves in the two resonant cavities are opposite, and this situation is defined as the out-of-phase mode, which is represented by the letter "Ak", where the number k represents the number of half-waves contained in the length of the resonant cavity. For example, "A1" means that exactly one half-wavelength is accommodated in the resonant cavity, the sound pressure reaches the maximum at the center of the cavity, and the sound pressure at the two ends' nodes is zero. It is the lowest-order out-of-phase mode, also known as the fundamental mode. The mainstream dual-channel differential photoacoustic cells all operate in this mode. "A2" means that there are two sound pressure bellies (extremes of the sound pressure signal) in the resonant cavity, and there is a node (the sound pressure signal is 0) in the middle. The length of the resonant cavity corresponds to a complete sound wave wavelength. And so on, the higher-order out-of-phase modes have more nodes and bellies. Since the acoustic energy of the higher-order modes is distributed on more nodes and bellies, and various acoustic dissipations attenuate the higher-order modes more severely, resulting in a decrease in the local sound pressure amplitude. In the case of only using two microphones to detect the sound pressure signal, the higher-order sound pressure signal differs greatly from the sound pressure signal in the fundamental mode. In addition, to eliminate the influence of in-phase noises such as environmental noise, airflow noise, and electromagnetic interference on the signal-to-noise ratio of the system, the photoacoustic cell should be selected to operate in the out-of-phase mode.

[0037] For the dual-channel differential photoacoustic cell of the present invention, in the out-of-phase mode, the sound pressure modal distribution function in the resonant cavity can be expressed as: x ∈ [0, L], where L is the resonant cavity and k is the number of half-waves contained in the length of the resonant cavity; after the shape, size, and order of the acoustic mode of the photoacoustic cell are determined, A and cos(ωt) remain unchanged. The only factor affecting the magnitude of the detected sound pressure signal is the position of the microphone, that is, the magnitude of x. According to the properties of trigonometric functions, when m is an odd number, the sound pressure signal will reach the maximum in this order mode, thereby determining the position of the microphone.

[0038] To ensure the effective extraction of the sound pressure signal in the high-order acoustic mode, the present invention sets strict matching rules for the number and position of microphones. The installation position of the microphone must be at the sound pressure extreme point (i.e., the sound pressure abdomen) in the corresponding mode. From the above analogy, the number of microphones installed on each resonant cavity of the present invention is the same as the order k. The installation position of the microphone is expressed by the formula:

[0039]

[0040] where k is the order k of Ak in the multi-order anti-phase mode, representing the number of half-waves included within the length of the resonant cavity, x1 is the position of the first microphone from left to right in the resonant cavity, x n is the position of the nth microphone from left to right in the resonant cavity, n is a positive integer and n ≤ k, and L is the cavity length of the resonant cavity.

[0041] As Figure 2 shown, the present invention provides a gas detection device based on multi-order acoustic modes, including: a signal generation and processing device and a differential resonant photoacoustic cell based on multi-order acoustic modes. The differential resonant photoacoustic cell based on multi-order acoustic modes is used to detect the sound signals of the gas to be measured in two resonant cavities; the signal generation and processing device is used to detect the concentration of the gas according to the sound signals of the gas. The signal generation and processing device includes a differential amplifier circuit, a first lock-in amplifier, a second lock-in amplifier, a first laser, a second laser, a first function generator, a second function generator, and a processor.

[0042] Refer to Figure 2 In Figure 2 Figure 21 is the differential resonant photoacoustic cell based on multi-order acoustic modes of the present invention, and 24 is the signal generation and processing device. 15 is the first laser, 16 is the first function generator, 17 is the first lock-in amplifier, 18 is the second laser, 19 is the second function generator, 20 is the second lock-in amplifier, 22 is a differential amplifier circuit applicable to multiple microphones, and 23 is the processor.

[0043] The first laser and the second laser are used to send laser signals to their corresponding resonant cavities;

[0044] The differential amplifier circuit is used to differentially amplify the sound signals detected by the microphones on each resonant cavity and send the amplified sound signals to the corresponding lock-in amplifier;

[0045] The first function generator and the second function generator are used to send a synchronization signal corresponding to the amplified acoustic signal to the corresponding lock-in amplifier; wherein, the frequency of the synchronization signals sent by the first function generator and the second function generator is one-half of the resonance frequency of the corresponding resonant cavity or the same as the resonance frequency of the resonant cavity, which is specifically determined by the primary or secondary harmonic demodulation parameters of the lock-in amplifier.

[0046] The first lock-in amplifier and the second lock-in amplifier are used to demodulate the signals received in the multi-order inverting mode and send the demodulated signals into the processor;

[0047] The processor is used to perform data processing on the demodulated signals to obtain the concentration of the gas.

[0048] The acoustic signals detected by the n microphones installed on the two resonant cavities in the multi-order inverting mode are expressed by the formula:

[0049]

[0050] In the formula, U 1kn is the acoustic signal detected by the nth microphone on the first resonant cavity, where the subscript 1 represents the first resonant cavity, k is the order k of Ak in the multi-order inverting mode, and n represents the nth microphone. A Ak is the signal amplitude in the Ak inverting mode, B n (t) is the total noise of the acoustic signal detected by the nth microphone (including gas flow noise, window noise, electromagnetic interference noise, external environmental noise, etc.), x 1n represents the installation position of the nth microphone on the first resonant cavity, x 2n represents the installation position of the nth microphone on the second resonant cavity, L is the cavity length of the resonant cavity, ω is the angular frequency, is the phase difference, U 2kn is the acoustic signal detected by the nth microphone on the second resonant cavity, where the subscript 2 represents the first resonant cavity, k is the order k of Ak in the multi-order inverting mode, and n represents the nth microphone. The order of the microphones on the first resonant cavity is 11, 12,... 1n, and the order of the microphones on the second resonant cavity is 21, 22,... 2n. Among them, the subscripts 1 and 2 in x 1n and x 2n are only used to distinguish the first and second resonant cavities, and the values of x 1n and x 2n can be represented by the above microphone position formula x n .

[0051] The amplified acoustic signal output by the differential amplifier circuit is expressed by the formula:

[0052] U = |[S 1k1+B1(t)] - [S 2k1 +B1(t)]| + |[S 1k2 +B2(t)] - [S 2k2 +B2(t)]| + …

[0053] +|[S 1kn +B n (t)] - [S 2kn +B n (t)]| = 2x·|S 1k1 |

[0054] The relationship between the photoacoustic signal intensity S after demodulation by the photoacoustic cell and the absorption coefficient α of the target gas is as follows:

[0055] S ∝ αPQ / f

[0056] In the formula, P is the laser power, Q is the quality factor, and f is the resonance frequency. The absorption coefficient α of the target gas can be calculated from the measured photoacoustic signal intensity S, and the relationship between the absorption coefficient α of the target gas and the gas concentration N is as follows:

[0057] α = σN

[0058] In the formula, σ is the gas absorption cross-section. Substituting the absorption coefficient α of the target gas into the above formula, the gas concentration information of the target gas can be obtained.

[0059] Next, according to the calculation principle of the differential amplifier circuit, the process of the sound signals detected by multiple microphones is described. Through acoustic theory, the sound signal in the photoacoustic cell under ideal conditions can be deduced and approximately expressed by the formula:

[0060] S(x, t) ∝ p(x, t) = A·φ k (x)·cos(ωt)

[0061] This formula is also the expression of the standing wave mode in the photoacoustic cell. A is the sound pressure amplitude, and the sound pressure amplitudes in different modes are different. φ k (x) is the sound pressure mode function, and cos(ωt) is the time term. During the detection process of the actual system, there will be noise interference, and the sound signal detected by the microphone can be expressed as:

[0062] U = S(x, t) + B(t)

[0063] where B(t) is the noise term.

[0064] For a dual-channel differential photoacoustic cell, in the inverting mode, the sound pressure mode function x ∈ [0, L], where L is the resonant cavity and k is the number of half - wavelengths contained within the length of the resonant cavity; after the shape of the photoacoustic cell and the order of the acoustic mode are determined, A and cos(ωt) remain unchanged. The only factor that affects the magnitude of the detected sound pressure signal is the position of the microphone, that is, the magnitude of x. According to the properties of trigonometric functions, when When m is odd, the sound pressure signal will reach its maximum in this order mode, thus determining the position of the microphone.

[0065] According to the above theory, when the photoacoustic cell operates in the A1 mode, It can be seen from the formula that when and only when the detected sound pressure signal is the largest, which is consistent with the above theory "A1 means that exactly one half - wavelength is accommodated in the resonant cavity, the sound pressure reaches its maximum at the center of the cavity, and the sound pressure at both ends is zero". The sound signals detected by the two microphones can be expressed by the following formulas respectively:

[0066]

[0067] Since the two resonant cavities are exactly the same and operate in the anti - phase mode, their phase difference Calculated by the formula, we get S 111 =-S 211 , S 111 and S 211 have equal amplitudes but opposite phases. The sound signals detected by the two microphones contain the same noise term B(t). After passing through the differential circuit output, the sound signal is U = |U 111 -U 211 | = |[S 111 +B1(t)] - [S 211 +B1(t)]| = 2·|S 111 |, which not only realizes signal amplification but also suppresses noise.

[0068] According to the theory, installing multiple microphones will not destroy the acoustic mode in the cavity or change the strength of the sound pressure. Taking six microphones as an example, the number of six microphones corresponds to the A3 mode of the dual - channel differential photoacoustic cell. According to the formula It can be solved that When the sound pressure signal reaches its maximum. Therefore, the three microphones of the first resonant cavity are installed at x 11 , x 12 , x 13 where The detected sound signals are:

[0069]

[0070] The three microphones of the second resonant cavity are installed at x 11 , x22 、 at x 23 where the detected acoustic signals are:

[0071]

[0072] The A3 mode is also an inverting mode, and its phase difference can be calculated by the formula to get S 131 =-S 231 , s 132 =-S 232 , S 133 =-S 233 , and their signal amplitudes are all equal. The signals detected by two microphones at the same longitudinal position are in opposite phases and contain the same noise terms. Finally, after being output by three differential amplifier circuits and then superimposed, we get:

[0073] U = |[S 131 +B1(t)] - [S 231 +B1(t)]| + |[S 132 +B2(t)] - [S 232 +B2(t)]|

[0074] + |[S 133 +B3(t)] - [S 233 +B3(t)]| = 6·|S 131 |

[0075] Thus, a six-fold amplification of the sound pressure signal in this order mode is achieved, and the noise is suppressed.

[0076] In the actual design process, since it is difficult to calculate the sound pressure signal, the finite element analysis software COMSOL can be used to perform acoustic simulation on the photoacoustic cell. This software can provide the analysis of the sound field distribution, and the accuracy of this software has been verified by many literatures. Through the simulation, it can be more intuitively seen the acoustic modes of each order in the photoacoustic cell, the variation of the sound pressure signal with the position change in the cavity, and the magnitude of the sound pressure signal, which provides a reference for the selection of acoustic modes and the arrangement of microphones.

[0077] The dual-channel differential photoacoustic cell of the present invention should be selected to operate in the anti-phase mode. The dual-channel differential photoacoustic cell mostly operates in the A1 mode. There is only one sound pressure extreme value in a resonant cavity, and it is located at the longitudinal center position. Therefore, the microphone is placed here, and then the sound pressure signal is amplified twice in this mode through a differential amplification circuit. According to the solution of the present invention, when the dual-channel differential photoacoustic cell is in high-order modes such as A2, A3, A4, A5, etc., the change of the sound pressure at the longitudinal position in the resonant cavity is determined by the sound pressure modal function. This function will have multiple corresponding sound pressure extreme values at different orders. The number of microphones is determined according to the number of sound pressure extreme values, and the position of the microphone can also be calculated by the formula. Then, the arrangement scheme of the microphones is comprehensively considered according to the results of the sound pressure signal magnitudes of each order in the software simulation. According to the formula, in the A2 mode, there are two sound pressure extreme values in a resonant cavity, which are located at the and of the resonant cavity. After being output through the differential circuit, the sound pressure signal in this mode can be amplified four times; in the A3 mode, there are three sound pressure extreme values in a resonant cavity, which are located at the and of the resonant cavity. After being output through the differential circuit, the sound pressure signal in this mode can be amplified six times.

[0078] In addition, in the anti-phase modes of Ak (k is odd) such as A1, A3, A5, etc., due to the symmetry of the mode, the sound pressure signal at the center position of the resonant cavity must be an extreme value. Therefore, when detecting multiple gases, this mode can be given priority to reduce the complexity of the microphone arrangement.

[0079] For example, when detecting two gases, using the A1 and A3 modes, the six microphone arrangement positions in the A3 mode can be shared, and there is no need to add additional microphones for the A1 mode. This not only realizes the effective acquisition of multi-modal signals but also improves the compactness of the system structure and the convenience of implementation. In this solution, the sound pressure signal in the A3 mode is amplified six times, and to a certain extent, the sound pressure signal in the A1 mode is also improved. Since different acoustic modes are generated at different excitation frequencies and the different excitation frequencies do not interfere with each other, simultaneous high-precision measurement of two gases can be achieved. Using this invention improves the utilization rate of acoustic modes, makes up for the shortcoming of low sound pressure signals in high-order modes, and realizes simultaneous high-precision measurement of multiple gases while having a simple structure.

[0080] Through theoretical calculation and simulation of the acoustic modes of the photoacoustic cell, the photoacoustic cell in this example includes a resonant cavity with a length of 90 mm and a radius of 4 mm, and buffer cavities with a length and radius of 10 mm are connected to both ends respectively. A quartz window with a thickness of 5 mm and a diameter of 25.4 mm is installed on one side of each buffer cavity away from the resonant cavity to achieve the sealing function. Since both the length and radius of the resonant cavity will affect its resonance frequency, the size parameters of the resonant cavity and buffer cavity involved in this patent are only preferred embodiments, and the actual structural dimensions can be adjusted according to specific application requirements, not limited to the above values. The acoustic mode simulation nephogram is as Figure 3 shown, where the sound pressure color from blue to red indicates that the sound pressure is getting higher and higher.

[0081] When measuring two gases, two acoustic modes, A1 and A3, are selected, and the installation positions of the six microphones are as Figure 2 shown. Microphones 9 and 12 are located at L / 6 of the resonant cavity, microphones 10 and 13 are located at L / 2, and microphones 11 and 14 are located at 5L / 6, where L is the length of the resonant cavity. Microphones 9, 12 and 11, 14 are symmetric about the central vertical axis of the resonant cavity. The positions of these microphones are obtained through theoretical calculation and correspond to the sound pressure extreme points in the A1 and A3 modes of the dual-channel differential photoacoustic cell. Figure 4 is the sound signal detected by the differential amplification circuit using only two microphones in the photoacoustic cell under the same conditions, Figure 5 is the frequency response curve diagram after differential amplification using six microphones in the present invention. By comparison, it can be obtained that the amplitude of the A1 mode signal using two microphones is three times that of the A3 mode signal. By using six microphones, the amplitude of the A3 mode sound signal of the same photoacoustic cell is amplified three times, and its intensity is equivalent to that of the A1 signal using two microphones. At the same time, the amplitude of the A1 mode signal using six microphones is also increased to a certain extent. The strength of the sound signal is related to the accuracy of gas detection. Through this example, the detection accuracy of the photoacoustic spectroscopy gas detection system under the same conditions can be improved by 2-3 times.

[0082] In addition, when measuring three gases, three acoustic modes, A1, A2, and A3, are selected. Since the sound pressure node at the center of the resonant cavity in the A2 mode is exactly zero, in order to detect the acoustic responses of all three modes simultaneously, the multi-microphone dual-channel differential photoacoustic cell design is as Figure 6 shown. In this embodiment, Figure 1On the basis of [the original configuration], four microphones are added. Among them, microphones 25 and 26 are located at the L / 4 position of the resonant cavity, and microphones 27 and 28 are located at the 3L / 4 position of the resonant cavity. They are also symmetric about the central vertical axis of the resonant cavity and correspond to the sound pressure extreme points in the A2 mode. Three lasers are used respectively corresponding to the absorption line wavelengths of three gases. Among them, two lasers are combined and enter one side of the resonant cavity of the photoacoustic cell, and the other laser is incident on the other side of the resonant cavity, and it is ensured that the laser completely passes through the photoacoustic cell without irradiating the wall surface of the photoacoustic cell. The three lasers are modulated with the resonance frequencies of the A1, A2, and A3 modes respectively. The subsequent operations are similar to the example with six microphones. Since the A1, A2, and A3 modes are independent of each other and have a high frequency discrimination degree. The acoustic signals of the two gases are collected simultaneously in the differential output and demodulated by three lock-in amplifiers at their respective frequencies. The acoustic responses between the two resonant cavities do not interfere with each other, and the system can realize the synchronous and independent detection of the concentrations of three gases.

[0083] It should be noted that the terms "first" and "second" in the present invention are only used for descriptive purposes and cannot 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, "a plurality" means two or more unless otherwise specifically defined.

[0084] Although the present application has been described in conjunction with various embodiments herein, however, in the process of implementing the claimed present application, those skilled in the art can understand and achieve other variations of the disclosed embodiments by viewing the accompanying drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other components or steps, and "a" or "one" does not exclude a plurality of cases.

[0085] The above content is a further detailed description of the present invention in combination with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is only limited to these descriptions. For those of ordinary skill in the technical field to which the present invention pertains, without departing from the concept of the present invention, several simple deductions or substitutions can be made, and all should be regarded as belonging to the protection scope of the present invention.

Claims

1. A high-precision differential resonance photoacoustic cell based on multi-order acoustic modes, characterized in that It includes two resonant cavities and two buffer cavities. The two resonant cavities are arranged in parallel vertically. One buffer cavity is provided at each end of the two resonant cavities. An optical window is installed and sealed on one side of each buffer cavity away from the resonant cavity. An air path inlet and outlet pipe is provided on each buffer cavity. A plurality of microphones are arranged outside each resonant cavity, and the plurality of microphones are arranged at the sound pressure extreme value points in the corresponding mode in the resonant cavity.

2. The high-precision differential resonant photoacoustic cell based on multi-order acoustic modes according to claim 1, wherein The microphone is installed in an installation groove, and the installation grooves are evenly arranged on the top of the resonant cavity.

3. The high-precision differential resonance photoacoustic cell based on multi-order acoustic modes according to claim 1, characterized in that The high-precision differential resonant photoacoustic cell based on multi-order acoustic modes has two modes, namely multi-order in-phase mode and multi-order anti-phase mode. The high-precision differential resonant photoacoustic cell based on multi-order acoustic modes operates in the multi-order anti-phase mode.

4. The high-precision differential resonant photoacoustic cell based on multi-order acoustic modes according to claim 1, wherein The installation quantity of the microphones on each resonant cavity is related to the order of the multi-order anti-phase mode, and the installation position and quantity of the microphones are determined by the position and quantity of the sound pressure extreme values.

5. The high-precision differential resonant photoacoustic cell based on multi-order acoustic modes according to claim 4, wherein The installation quantity of the microphones on each resonant cavity is the same as the order k, and the installation position of the microphones is expressed by the formula: Among them, k is the order number k of Ak in the multi-order inverse phase mode, representing the number of half-waves contained within the resonator length, x1 is the position of the first microphone from left to right in the resonator, x n is the position of the nth microphone from left to right in the resonator, n is a positive integer and n ≤ k, and L is the cavity length of the resonator.

6. A gas detection device based on multi-order acoustic modes, characterized in that, It includes: A signal generating and processing device and the differential resonant photoacoustic cell based on multi-order acoustic modes according to any one of claims 1-5. The differential resonant photoacoustic cell based on multi-order acoustic modes is used to detect the sound signals of the gas to be measured in the two resonant cavities; the signal generating and processing device is used to detect the concentration of the gas according to the sound signals of the gas.

7. The gas detection device based on multi-order acoustic modes according to claim 6, characterized in that, The signal generating and processing device includes a differential amplifier circuit, a first lock-in amplifier, a second lock-in amplifier, a first laser, a second laser, a first function generator, a second function generator and a processor; The first laser and the second laser are used to send laser signals to their corresponding resonant cavities; The differential amplifier circuit is used to differentially amplify the sound signals detected by the microphones on each resonant cavity and send the amplified sound signals to the corresponding lock-in amplifier; The first function generator and the second function generator are used to send synchronization signals synchronized with the amplified sound signals to their corresponding lock-in amplifiers; The first lock-in amplifier and the second lock-in amplifier are used to demodulate the signals received in the multi-order anti-phase mode and send the demodulated signals into the processor; The processor is used to perform data processing on the demodulated signals to obtain the concentration of the gas.

8. The gas detection device based on multi-order acoustic modes according to claim 7, characterized in that The frequency of the synchronization signals sent by the first function generator and the second function generator is one-half of the resonance frequency of the corresponding resonant cavity or the same as the resonance frequency of the resonant cavity.

9. The gas detection device based on multi-order acoustic modes according to claim 7, wherein The sound signals detected by the n microphones installed on the two resonant cavities in the multi-order anti-phase mode are expressed by the formula: Where U 1kn is the acoustic signal detected by the nth microphone on the first resonator. The subscript 1 represents the first resonator, k is the order k of Ak in the multi - order antiphase mode, n represents the nth microphone, and A Ak is the signal amplitude in the Ak antiphase mode. B n (t) is the total noise of the acoustic signal detected by the nth microphone. x 1n represents the installation position of the nth microphone on the first resonator. x 2n represents the installation position of the nth microphone on the second resonator. L is the cavity length of the resonator, ω is the angular frequency, is the phase difference. U 2kn is the acoustic signal detected by the nth microphone on the second resonator. The subscript 2 represents the first resonator, k is the order k of Ak in the multi - order antiphase mode, and n represents the nth microphone. The order of microphones on the first resonator is 11, 12, … 1n, and the order of microphones on the second resonator is 21, 22, … 2n. The subscripts 1 and 2 in x 1n and x 2n are only used to distinguish the first and second resonators.

10. The gas detection device based on multi-order acoustic modes according to claim 9, wherein, The amplified sound signals output by the differential amplifier circuit are expressed by the formula: U = |[S 1k1 + B1(t)] - [S 2k1 + B1(t)]| + |[S 1k2 + B2(t)] - [S 2k2 + B2(t)]| + … + |[S 1kn + B n (t)] - [S 2kn + B n (t)]| = 2x · |S 1k1 |。

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