A concentration detection method based on laser multi-gas aliasing spectrum demodulation and nonlinear compensation

Abstract

The present invention discloses a concentration detection method for laser multi-gas overlapping spectrum demodulation and nonlinear compensation, comprising the following steps: First, collect the second harmonic signals of the gas to be measured, and demodulate its severely overlapping spectrum signals based on the non-negative least squares method, so as to obtain the concentration results when measuring multiple gases. However, due to the generation of nonlinear effects, a cubic polynomial is used to compensate the concentration results, and more accurate concentration measurement results of multiple gases are obtained. Taking CO and CH4 in coal mines as examples for illustration. Through the above method, the present invention can realize the demodulation of the second harmonic overlapping spectrum of multiple gases, and perform nonlinear compensation on the concentration based on the demodulated results to obtain accurate gas concentration information, meeting the requirements of simultaneous on-line monitoring of multiple gases in special working condition environments.

Description

Technical Field

[0001] The invention relates to the technical field of gas detection, and in particular to a concentration detection method of laser multi-gas aliasing spectrum demodulation and nonlinear compensation. Background Art

[0002] When using laser spectroscopy technology to measure multiple gases simultaneously, the problem of cross-interference between spectral lines has always been a troubling problem. For example, CO and CH4 gases are two common gases in the coal mine production environment. CO gas has a very strong affinity with human hemoglobin, which will greatly reduce the ability of hemoglobin to transport oxygen, leading to the occurrence of poisoning symptoms. CH4 gas, also known as methane, is the main gas produced in coal mining. It is a flammable and explosive gas that seriously threatens the production safety of coal mines. Therefore, it is very necessary to conduct real-time online high-precision detection of CO and CH4 gases in coal mines. Under normal circumstances, the allowable CO concentration in mines does not exceed 24ppm, and the allowable CH4 concentration generally does not exceed 5×10 4 ppm. It can be seen that the concentrations of the two differ by 3 or even 4 orders of magnitude.

[0003] Electrochemical sensors are a common method for detecting CO and CH4 in coal mines. Independent electrochemical sensors must be selected for the two gases. However, during use, electrochemical sensors do not have good selectivity, are prone to poisoning and aging, have a short service life, and generally need to be replaced every six months or a year. Their measurement accuracy is also not high. Another commonly used method is gas chromatography technology, which is complex to operate, has a large equipment volume, a long analysis cycle, and is difficult to achieve real-time in-situ online detection. Therefore, traditional methods cannot achieve real-time online, high-precision simultaneous detection of trace CO and high-concentration CH4 in coal mines.

[0004] Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology uses a narrow linewidth tunable laser as a light source, and obtains a suitable laser output wavelength by controlling temperature and current. When the laser passes through a gas medium that absorbs the wavelength, the energy is absorbed. According to the Beer-Lambert absorption law, the gas absorption intensity on the beam path can be obtained by measuring the transmission spectrum signal, and then the gas concentration parameters can be inverted. In practical applications, in order to measure trace gases with lower concentrations, wavelength modulation technology is often used. Wavelength modulation technology has the advantages of strong anti-interference ability and high sensitivity, but wavelength modulation technology generally requires that the measured gas absorbance be less than 0.05, otherwise it will produce obvious nonlinearity. In coal mines, the absorbance of trace CO will not exceed 0.05, but the absorbance of high-concentration CH4 will often far exceed 0.05. In addition, during the measurement process, the second harmonic signal of trace CO will be seriously interfered by the second harmonic signal of the high-concentration CH4 mixed gas, resulting in the inability to accurately measure the CO concentration. The Chinese Academy of Sciences and other related research institutions have applied wavelength modulation technology to the concentration detection of gases such as CO and CO2. There are also related literature on the measurement of multiple gases under working conditions such as coal mines, but none of them discusses the spectral demodulation and concentration inversion under conditions where the concentrations of two or more gases are very different. There are no literature reports on demodulating the aliased spectral signals of trace CO and high-concentration CH4 gas (concentration difference of 3-4 orders of magnitude) in special working conditions such as coal mines, and measuring the concentrations of the two gases at the same time.

[0005] In special working conditions, multiple gases exist at the same time, and the concentration difference is 3-4 orders of magnitude. For example, in coal mines, trace CO and high-concentration CH4 exist at the same time, and high-concentration CH4 will seriously interfere with the signal of trace CO, which makes it difficult to measure both using the same system.

[0006] Therefore, there is an urgent need to provide a new method for simultaneously detecting multiple gases to solve the above problems. Summary of the invention

[0007] The technical problem to be solved by the present invention is to provide a concentration detection method for laser multi-gas aliasing spectrum demodulation and nonlinear compensation, which can realize the demodulation of second harmonic aliasing spectra of multiple gases, and perform nonlinear compensation on the concentration based on the demodulation results to obtain accurate gas concentration information.

[0008] In order to solve the above technical problems, a technical solution adopted by the present invention is: to provide a concentration detection method of laser multi-gas aliasing spectrum demodulation and nonlinear compensation, wherein the gas to be detected is a multi-gas aliasing spectrum signal, including gas A and gas B, and gas A is a single spectral line absorption gas with an absorbance less than 0.05, and the absorbance of gas B is greater than 0.05. The detection method comprises the following steps:

[0009] S1: Collect the second harmonic of standard gas A and gas B as reference signal. The concentration of gas A and gas B at this time is recorded as CR 气体A , CR 气体B , the second harmonic signal is recorded as vector SR 气体A , SR 气体B ;

[0010] S2: collect the real-time second harmonic signal of the gas to be tested, denoted as S;

[0011] S3: Based on the real-time second harmonic signal S in step S2 and the gas A and gas B reference signals SR in step S1 气体A , SR 气体B Based on the non-negative least squares method, the concentrations of gas A and gas B are CM 气体A CM 气体B ;

[0012] S4: Collect the second harmonic signals of four groups of gas B with different standard concentrations C1, C2, C3, and C4, and calculate the concentrations of these four groups of gas B using the least squares method, which are recorded as M1, M2, M3, and M4;

[0013] S5: Perform cubic polynomial fitting on the four groups of standard concentrations C1, C2, C3, C4 and the four groups of calculated concentrations M1, M2, M3, M4 in step S4 to obtain a nonlinear correction model for the concentration of gas B;

[0014] S6: The concentration CM of the gas B obtained in step S3 is corrected according to the nonlinear correction model in step S5. 气体B Correction is performed to obtain the final gas B concentration C 气体B ;

[0015] S7: According to the proportional relationship between the concentrations of gas B before and after correction, the concentration of gas A in the mixed gas is corrected to obtain the final concentration C of gas A. 气体A .

[0016] In a preferred embodiment of the present invention, in steps S1, S2 and S4, the second harmonic signals of the collected gas are subjected to data preprocessing, including background light intensity elimination, spectrum correction, multiple averaging, and SG filtering.

[0017] In a preferred embodiment of the present invention, in step S3, the concentrations CM of gas A and gas B in the real-time second harmonic signal are calculated. 气体A CM 气体B The specific steps include:

[0018] First, the real-time second harmonic signal S is recorded as the proportional superposition of the second harmonics of the standard gas A and gas B in step S1, expressed as follows:

[0019] S=k1×SR 气体A +k2×SR 气体B =[k1, k2] T [SR 气体A , SR 气体B ]

[0020] Among them, k1 and k2 are proportional coefficients;

[0021] Then, the proportional coefficients k1 and k2 are solved based on the non-negative least squares method. The formula is as follows:

[0022] min β ||[k1, k2] T [SR 气体A , SR 气体B ]-S||2 2 stk1≥0,k2≥0;

[0023] Finally, the concentrations CM of gas A and gas B in the real-time second harmonic signal are calculated according to the following two equations: 气体A CM 气体B :

[0024] CM 气体A =k1×CR 气体A

[0025] CM 气体B =k2×CR 气体B .

[0026] In a preferred embodiment of the present invention, the specific steps of step S4 include:

[0027] The second harmonic signals of gas B with four different standard concentrations of C1, C2, C3, and C4 are linearly fitted with the reference signal of gas B in step S1 to obtain the coefficient g i (where i = 1, 2, 3, 4), and then the corresponding results are M1, M2, M3, and M4 according to the following formula:

[0028] M i =g i ×CR 气体B , i=1,2,3,4.

[0029] In a preferred embodiment of the present invention, in step S5, the specific steps of obtaining the nonlinear correction model of the concentration of gas B include:

[0030] According to the Taylor expansion formula, a cubic polynomial is used for fitting, and the final nonlinear correction model is expressed as:

[0031] M=f0+f1×C+f2×C 2 +f3×C 3

[0032] Wherein, M represents the calculated concentration result, C represents the concentration of the standard gas, and f0, f1, f2, and f3 are the 0th, 1st, 2nd, and 3rd order coefficients of the cubic polynomial, respectively.

[0033] In a preferred embodiment of the present invention, in step S7, the formula for correcting the concentration of gas A in the mixed gas is as follows:

[0034]

[0035] The principle of completing aliasing spectrum demodulation in the present invention is based on a non-negative least squares algorithm, and the nonlinear compensation is based on the principle of Beer-Lambert's law and wavelength modulation technology and its Taylor expansion formula.

[0036] The beneficial effects of the present invention are:

[0037] (1) The present invention can realize the demodulation of the second harmonic aliasing spectra of multiple gases, and perform nonlinear compensation on the concentration based on the demodulation results to obtain accurate gas concentration information; this method can make up for the shortcomings of the existing methods and meet the needs of simultaneous online monitoring of multiple gases in special working conditions;

[0038] (2) The present invention uses a wavelength modulation system to complete the simultaneous online monitoring of CO and CH4 in coal mines. The innovation of this method lies in: 1. The demodulation of the second harmonic aliasing spectrum of the two gases is completed by using the non-negative least squares method; 2. The wavelength modulation technology is explored and it is found that when measuring gas concentration, if the gas absorbance exceeds 0.05, according to the Taylor expansion formula, it is necessary to introduce the subsequent quadratic terms, cubic terms or even higher-order terms to meet the needs of accurate measurement. Based on this, the present invention uses a cubic polynomial to perform nonlinear compensation for the concentration. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a flow chart of a concentration detection method of laser multi-gas aliasing spectrum demodulation and nonlinear compensation of the present invention;

[0040] Figure 2 It is a schematic diagram of the second harmonic signal of single CO gas, single CH4 gas, and a mixture of the two;

[0041] Figure 3 It is a schematic diagram of the nonlinear correction model of CH4 concentration. DETAILED DESCRIPTION

[0042] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making a clearer and more definite definition of the protection scope of the present invention.

[0043] See also Figure 1 and Figure 2 , the embodiment of the present invention includes:

[0044] A concentration detection method for laser multi-gas aliasing spectrum demodulation and nonlinear compensation comprises the following steps:

[0045] This example takes the aliased spectral signals of trace CO and high-concentration CH4 gas (the concentration difference is 3-4 orders of magnitude) in special working conditions such as coal mines as an example. Similarly, the concentration detection method of the present invention is also applicable to other multi-gas aliased spectral signals, wherein there is a trace (volume fraction) gas (A) with a gas absorbance less than 0.05 and a high-concentration gas (B) with a gas absorbance far exceeding 0.05. The trace gas is interfered by the high-concentration gas, as shown in the following table:

[0046]

[0047]

[0048] S1: Collect the second harmonic of CO and CH4 standard gases as reference signals. The concentrations of CO and CH4 at this time are recorded as CR CO , CR CH4 , the second harmonic signal is recorded as vector SR CO , SR CH4 ;

[0049] Among them, the second harmonic signals of CO and CH4 standard gases with known concentrations were collected and data preprocessed, including background light intensity elimination, spectrum correction, multiple averaging, and SG filtering. The second harmonic signals collected in the following steps S2 and S4 were subjected to the same data preprocessing.

[0050] S2: collect the real-time second harmonic signal of the gas to be tested, denoted as S;

[0051] Under certain conditions, it can be understood that the second harmonic signal S obtained at this time is the proportional superposition of the second harmonics of the standard CO and CH4 gases in step S1, which can be expressed as follows:

[0052] S=k1×SR CO+k2×SR CH4 =[k1, k2] T [SR CO , SR CH4 ]

[0053] Among them, k1 and k2 are proportional coefficients.

[0054] S3: Based on the real-time second harmonic signal S in step S2 and the CO and CH4 reference signals SR in step S1 CO , SR CH4 Based on the non-negative least squares method, the concentrations of CO and CH4 are CM CO CM CH4 ;

[0055] The non-negative least squares method is a constrained linear regression method for solving the two proportional coefficients k1 and k2 in step S2, which can be expressed as:

[0056]

[0057] After using this method to solve the values ​​of the two proportional coefficients, the concentrations of CO and CH4 can be calculated using the following two equations.

[0058] CM CO =k1×CR CO

[0059] CM CH4 =k2×CR CH4

[0060] S4: Collect the second harmonic signals of four groups of CH4 gases with different standard concentrations C1, C2, C3, and C4, and calculate the concentrations of the four groups of CH4 gases by the least square method, which are recorded as M1, M2, M3, and M4. The specific steps include:

[0061] Collect four groups of second harmonic signals of CH4 gas with different standard concentrations (concentrations are C1, C2, C3, and C4, respectively), and perform the same data preprocessing as in step S1; use the least squares method to calculate the slope coefficients g1, g2, g3, and g4 for these four groups of signals and the CH4 reference signal in step 1, respectively, and then calculate the corresponding results M1, M2, M3, and M4 according to the following formula.

[0062] M i =g i ×CR CH4 i=1,2,3,4

[0063] S5: Perform cubic polynomial fitting on the four groups of standard concentrations C1, C2, C3, C4 and the four groups of calculated concentrations M1, M2, M3, M4 in step S4 to obtain a nonlinear correction model for CH4 concentration, see Figure 3 ; The specific steps include:

[0064] According to Taylor expansion formula Using a cubic polynomial for fitting can yield better results, and the final nonlinear correction model is expressed as:

[0065] M=f0+f1×C+f2×C 2 +f3×C 3

[0066] Wherein, M represents the calculated concentration result, C represents the concentration of the standard gas, and f0, f1, f2, and f3 are the 0th, 1st, 2nd, and 3rd order coefficients of the cubic polynomial, respectively.

[0067] S6: Using the nonlinear correction model obtained in step S5, the CH4 concentration CM calculated in step S3 is CH4 Substituting into the above formula, a more accurate CH4 concentration that is closer to the standard result can be calculated, which is the CH4 concentration to be measured and recorded as C CH4

[0068] S7: Based on the CH4 concentration CM before correction CH4 and the corrected CH4 concentration C CH4 The CO concentration in the mixed gas is corrected by the proportional relationship between them. The formula is as follows: CO .

[0069]

[0070] Since the error of the calculated CO concentration at this time mainly comes from the nonlinearity caused by the high concentration of CH4, the CO concentration can be corrected using the proportional relationship between the CH4 concentrations before and after correction to obtain a more accurate result.

[0071] The present invention uses the non-negative least squares method to process the second harmonic aliasing spectrum of the trace CO and CH4 mixed gas to obtain the concentration of each of them. Then, based on the nonlinearity of the second harmonic, a cubic polynomial is used to compensate the obtained concentration, and finally accurate gas concentration data is obtained.

[0072] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A concentration detection method using laser multi-gas aliasing spectrum demodulation and nonlinear compensation, characterized in that: The gas to be detected is a multi-gas mixed spectral signal, including gas A and gas B, and gas A is a single spectral line absorption gas with an absorbance less than 0.05, and the absorbance of gas B is greater than 0.

05. The detection method includes the following steps: S1: Collect the second harmonic of standard gas A and gas B as reference signal. The concentration of gas A and gas B at this time is recorded as CR 气体A , CR 气体B , the second harmonic signal is recorded as vector SR 气体A , SR 气体B ; S2: collect the real-time second harmonic signal of the gas to be tested, denoted as S; S3: Based on the real-time second harmonic signal S in step S2 and the gas A and gas B reference signals SR in step S1 气体A , SR 气体B Based on the non-negative least squares method, the concentrations of gas A and gas B are CM 气体A CM 气体B ; The specific steps include: First, the real-time second harmonic signal S is recorded as the proportional superposition of the second harmonics of the standard gas A and gas B in step S1, expressed as follows: S=k1×SR 气体A +k2×SR 气体B =[k1,k2] T [SR 气体A ,SR 气体B ] Among them, k1 and k2 are proportional coefficients; Then, the proportional coefficients k1 and k2 are solved based on the non-negative least squares method. The formula is as follows: my β ||[k1,k2] T [SR 气体A ,SR 气体B ]-S||2 2 stk1≥0,k2≥0; Finally, the concentrations CM of gas A and gas B in the real-time second harmonic signal are calculated according to the following two equations: 气体A CM 气体B : CM 气体A =k1×CR 气体A CM 气体B =k2×CR 气体B : S4: Collect the second harmonic signals of four groups of gas B with different standard concentrations C1, C2, C3, and C4, and calculate the concentrations of these four groups of gas B using the least squares method, which are recorded as M1, M2, M3, and M4; S5: Perform cubic polynomial fitting on the four groups of standard concentrations C1, C2, C3, C4 and the four groups of calculated concentrations M1, M2, M3, M4 in step S4 to obtain a nonlinear correction model for the concentration of gas B; the specific steps include: According to the Taylor expansion formula, a cubic polynomial is used for fitting, and the final nonlinear correction model is expressed as: M=f0+f1×C+f2×C 2 +f3×C 3 Wherein, M represents the calculated concentration result, C represents the concentration of the standard gas, f0, f1, f2, and f3 are the 0th, 1st, 2nd, and 3rd order coefficients of the cubic polynomial respectively; S6: The concentration CM of the gas B obtained in step S3 is corrected according to the nonlinear correction model in step S5. 气体B Correction is performed to obtain the final gas B concentration C 气体B ; S7: According to the proportional relationship between the concentrations of gas B before and after correction, the concentration of gas A in the mixed gas is corrected to obtain the final concentration C of gas A. 气体A , the corrected formula is as follows:

2. The concentration detection method of laser multi-gas aliasing spectrum demodulation and nonlinear compensation according to claim 1 is characterized in that: In steps S1, S2 and S4, the second harmonic signals of the collected gas are subjected to data preprocessing, including background light intensity elimination, spectrum correction, multiple averaging, and SG filtering.

3. The concentration detection method of laser multi-gas aliasing spectrum demodulation and nonlinear compensation according to claim 1 is characterized in that: The specific steps of step S4 include: The second harmonic signals of gas B with four different standard concentrations of C1, C2, C3, and C4 are linearly fitted with the reference signal of gas B in step S1 to obtain the coefficient g i , where i = 1, 2, 3, 4, and then the corresponding results are M1, M2, M3, M4 according to the following formula, M i =g i ×CR 气体B ,i=1,2,3,4。

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