CN113390825B - TDLAS-based time-frequency domain combined gas concentration inversion method and device - Google Patents
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Abstract
The invention provides a TDLAS-based time-frequency domain combined gas concentration inversion method, which comprises the steps of obtaining second harmonic signals of standard gas under different concentrations; extracting second harmonic signal peak values of the standard gas under different concentrations to obtain time domain characteristic points; decomposing second harmonic signals of the standard gas under different concentrations, and obtaining corresponding frequency spectrums through fast Fourier transform; extracting second harmonic frequency amplitude components of the standard gas under different concentrations to obtain frequency domain characteristic points; storing data of standard gas under different concentrations and establishing a gas concentration inversion model; and predicting the concentration of the gas to be detected by adopting a gas concentration inversion model. The gas concentration inversion method ensures the accuracy of the predicted gas concentration to be measured by combining the time domain characteristic points and the frequency domain characteristic points of the second harmonic signals containing the gas concentration information to invert the gas concentration.
Description
Technical Field
The invention relates to the technical field of gas concentration detection, in particular to a time-frequency domain combined gas concentration inversion method and device based on TDLAS.
Background
The tunable semiconductor laser absorption spectroscopy (TDLAS) technique is a spectroscopic measurement method that applies laser to an absorption spectroscopy measurement technique, and the principle of detecting gas concentration using it is: when the laser penetrates through the gas to be detected and the laser wavelength is the same as the central wavelength of the absorption spectrum line of the gas to be detected, the gas molecules to be detected absorb the laser with the wavelength, so that the intensity of the laser is attenuated, and the concentration of the gas to be detected is calculated by analyzing the light intensity information of the transmitted light.
However, the current photo-detector receives the light intensity signal of the transmitted light, which is very weak relative to the background signal and noise, and the detection accuracy of the gas concentration is seriously affected by the influence of the background environment. Meanwhile, when the gas concentration is detected and analyzed, the traditional detection method only analyzes from the angle of the time domain, and when the detected gas concentration changes a little, the second harmonic amplitude change in the time domain is also very small, so that the second harmonic amplitude change is difficult to accurately judge from the time domain, and the gas concentration measurement precision is influenced.
Disclosure of Invention
The invention aims to provide a time-frequency domain combined gas concentration inversion method and a time-frequency domain combined gas concentration inversion device based on TDLAS.
The technical scheme for realizing the purpose of the invention is as follows:
in a first aspect, the present invention provides a TDLAS-based time-frequency domain combined gas concentration inversion method, including the following steps:
and 7, collecting second harmonic signals of the gas to be detected, obtaining time domain characteristic points and frequency domain characteristic points of the gas to be detected, and predicting the concentration of the gas to be detected by adopting a gas concentration inversion model.
According to the gas concentration inversion method, the time domain characteristic points and the frequency domain characteristic points of the second harmonic signals of the standard gas with different concentrations are combined to invert the concentration of the gas to be measured, so that the accuracy of the predicted concentration of the gas to be measured is ensured, and the problems that the concentration of the gas to be measured is difficult to accurately judge from the time domain and the predicted value of the concentration of the gas to be measured is large in error and low in accuracy due to the fact that the concentration of the gas to be measured is difficult to accurately judge under the condition that the concentration of the gas to be measured is changed slightly and the second harmonic signals in the time domain are changed slightly are solved.
In one embodiment of the invention, the acquisition of the second harmonic signals of the standard gas and the gas to be detected is realized by a designed second harmonic signal acquisition system;
acquiring a second harmonic signal, comprising the following steps:
step 101, measuring the initial laser intensity S by the wavelength modulation technique 0 Modulating, outputting modulated laser light S 1 ；
Step 102, inputting modulated laser S 1 After the absorption of the standard gas or the gas to be measured, the transmitted light intensity signal S is output 2 ；
103, the transmitted light intensity signal S is processed 2 Performing superposition noise processing to output transmitted light intensity signal S 3 ；
104, utilizing the phase-locked amplification technology to transmit the light intensity signal S 3 And processing and outputting a second harmonic signal.
In one embodiment of the present invention, both the second harmonic signal peak value and the second harmonic frequency amplitude component are extracted by MATLAB algorithm.
In one embodiment of the present invention, in step 3, the second harmonic signal is decomposed by a discrete wavelet transform method.
In one embodiment of the present invention, in step 6, the gas concentration inversion model is established by a partial least squares regression method.
In a preferred embodiment of the present invention, the gas concentration inversion method further includes a step 8 of performing error analysis on the predicted concentration value of the gas to be measured by the RMSEP method.
In a second aspect, the present invention further provides a second harmonic signal acquisition system, which is used for acquiring and obtaining a second harmonic signal in the gas concentration inversion method in the first aspect. The second harmonic signal acquisition system comprises a light source control module, an air chamber absorption module, a noise module and a data processing module.
The light source control module modulates incident laser and outputs the modulated laser, the input of the light source control module is a low-frequency sawtooth wave signal, a high-frequency sine wave signal, initial laser intensity and laser center frequency, and the output of the light source control module is the frequency and light intensity of the modulated laser.
And the air chamber absorbs the standard air in the module or the modulated laser input by the pair to be detected, and then outputs a transmission light intensity signal.
The noise module is used for generating white noise and processing the superposed noise of the transmitted light intensity signal of the output air chamber absorption module.
The data processing module is used for processing the transmission light intensity signal of the superimposed noise and outputting the transmission light intensity signal as a second harmonic signal.
According to the invention, by designing a second harmonic signal acquisition system, the incident laser is modulated by superposing a high-frequency modulation signal on a low-frequency scanning signal, so that the detection precision of the system is improved; meanwhile, white noise generated by the noise module is superposed into a transmitted light intensity signal before the data processing module is input, so that low-frequency noise interference is avoided.
In a third aspect, the invention further provides a gas inversion apparatus, which includes the second harmonic signal acquisition system in the second aspect.
Compared with the prior art, the invention has the beneficial effects that:
1. the gas concentration inversion method realizes the prediction of the concentration of the gas to be measured by combining the time domain characteristic points and the frequency domain characteristic points, and ensures the accuracy of the predicted concentration of the gas to be measured.
2. By designing a second harmonic signal acquisition system, the incident laser is modulated by superposing a high-frequency modulation signal on a low-frequency scanning signal, so that the detection precision of the system is improved; meanwhile, white noise generated by the noise module is superposed into a transmission light intensity signal before the data processing module is input, so that low-frequency noise interference is avoided.
Drawings
In order to more clearly illustrate the technical solution of the embodiment of the present invention, the drawings used in the description of the embodiment will be briefly introduced below.
FIG. 1 is a flow chart of a TDLAS-based time-frequency domain combined gas concentration inversion method according to the present invention;
FIG. 2 is a flowchart of the TDLAS-based time-frequency domain combined gas concentration inversion method in example 1;
FIG. 3 is a design diagram of a second harmonic signal acquisition system;
fig. 4 is a schematic diagram of laser frequency and laser intensity output after incident laser is modulated by a light source control module;
FIG. 5 is a transmitted light intensity signal S output after the modulated laser is absorbed by methane gas (standard gas) in the gas chamber absorption module 2 A schematic diagram of variations;
FIG. 6 is a transmitted light intensity signal S output after noise is superimposed by a noise block 3 A schematic diagram of variations;
FIG. 7 is a schematic diagram of the second harmonic signal output by the signal processing module (i.e., the second harmonic signal acquisition system);
FIG. 8 is a schematic diagram of time domain feature points of a second harmonic signal;
FIG. 9a is a diagram illustrating detail components of d1 layer to d4 layer after discrete wavelet transform;
FIG. 9b is a diagram illustrating detail components of the corresponding d5 layer to d9 layer after discrete wavelet transform;
FIG. 9c is a diagram showing the detail components of the corresponding d10 layer to d14 layer after discrete wavelet transform;
fig. 10 is a schematic diagram of frequency domain characteristic points of a second harmonic signal.
Detailed Description
The invention will be further described with reference to specific embodiments, and the advantages and features of the invention will become apparent as the description proceeds. These examples are illustrative only and do not limit the scope of the present invention in any way. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention, and that such changes and substitutions are intended to be within the scope of the invention.
The specific embodiment provides a time-frequency domain combined gas concentration inversion method based on TDLAS, and as shown in fig. 1, the gas concentration inversion method includes the following steps:
The acquisition of the second harmonic signal of the standard gas is realized through a designed second harmonic signal acquisition system. The second harmonic signal acquisition system is designed by utilizing a Simulink (visual simulation tool) based on the principle of tunable semiconductor laser absorption spectroscopy (TDLAS), and comprises a light source control module, an air chamber absorption module, a noise module and a data processing module, wherein the principle is that incident laser is modulated by superposing a high-frequency modulation signal through a low-frequency scanning signal, and the detection precision of the system is improved.
Specifically, the light source control module modulates incident laser and outputs the modulated laser, and the input of the light source control module is a low-frequency sawtooth wave signal, a high-frequency sine wave signal, initial laser intensity and laser center frequency; meanwhile, white noise generated by the noise module is superposed into a transmitted light intensity signal before the data processing module is input, so that low-frequency noise interference is avoided. The output of the light source control module is the frequency and the light intensity of the modulated laser. The air chamber absorbs the modulated laser and outputs the transmitted light intensity signal. The noise module is used for generating white noise and processing the transmission light intensity signal of the output air chamber absorption module by superposing the noise. The data processing module is used for processing the transmission light intensity signal of the superimposed noise and outputting the transmission light intensity signal as a second harmonic signal.
Specifically, the acquisition of the second harmonic signal includes the following steps:
step 101, the initial laser intensity S is modulated by the wavelength modulation technique 0 Modulating, outputting modulated laser light S 1 ；
Step 102,Input modulated laser light S 1 And the transmitted light intensity signal S is output after the absorption of the standard gas 2 ；
Step 103, for the transmitted light intensity signal S 2 Processing the superposition noise and outputting a transmitted light intensity signal S 3 ；
104, utilizing the phase-locked amplification technology to transmit the light intensity signal S 3 And processing and outputting a second harmonic signal.
specifically, the method for extracting the second harmonic signal peak is an MATLAB algorithm.
specifically, the second harmonic signal is decomposed by a discrete wavelet transform method.
specifically, the method for extracting the second harmonic frequency amplitude component is an MATLAB algorithm.
specifically, the gas concentration inversion model is established by a partial least squares regression method.
And 7, collecting second harmonic signals of the gas to be detected, obtaining time domain characteristic points and frequency domain characteristic points of the gas to be detected, and predicting the concentration of the gas to be detected by adopting a gas concentration inversion model.
And 8, carrying out error analysis on the predicted concentration value of the gas to be detected by the RMSEP method.
The present embodiment also provides a gas inversion apparatus, including the second harmonic signal acquisition system in the second aspect.
According to the gas concentration inversion method, the time domain characteristic points and the frequency domain characteristic points of the second harmonic signals containing the gas concentration information are combined to invert the concentration of the gas to be measured, the accuracy of the predicted gas concentration to be measured is ensured, and the problems that the gas concentration to be measured is difficult to accurately judge from the time domain and the predicted value error of the gas concentration to be measured is large and the accuracy is low due to the fact that the second harmonic signals in the time domain are small when the concentration of the gas to be measured is changed a little are solved.
Example 1:
the TDLAS-based time-frequency domain combined gas concentration inversion method of the present invention is specifically described below by taking methane gas as a standard gas, as shown in fig. 2, and comprises the following steps:
s1, designing a second harmonic signal acquisition system by using Simulink, wherein the second harmonic signal acquisition system comprises a light source control module, an air chamber absorption module, a noise module and a data processing module, and acquiring and extracting a second harmonic signal through the second harmonic signal acquisition system. As shown in fig. 3, is a design diagram of a second harmonic signal acquisition system.
Specifically, the light source control module is used for controlling the incident laser (S) 0 ) Modulating the laser beam and outputting the modulated laser beam S 1 (ii) a The gas chamber absorption module simulates modulated laser S 1 Through the absorption process of the gas absorption cell, a transmitted light intensity signal S is output 2 (ii) a The noise module is used for outputting a transmitted light intensity signal S of the air chamber absorption module 2 Noise is superimposed to avoid interference of low noise, and a transmission light intensity signal S after noise addition is output 3 (ii) a The signal processing module is used for transmitting the light intensity signal S after the noise is superimposed 3 And processing and outputting a second harmonic signal.
S2, adopting the modulated laser to obtain an absorption spectral line (the wavelength is 1653.72nm, and the wave number is 6046.97 cm) of methane gas under a certain concentration -1 ) Scanning, using harmonicsThe wave detection technique extracts the second harmonic signal in the transmitted light.
Specifically, the process of acquiring the second harmonic signal is as follows:
firstly, inputting a low-frequency sawtooth wave signal (the tuning frequency is selected to be 1 Hz), a high-frequency sine wave signal (the modulation frequency is selected to be 2 kHz), initial laser intensity (5 mw) and laser center frequency (6046.97 cm < -1 >); the output of the light source control module is the modulated laser output frequency and laser output intensity, and the schematic diagram of the modulated laser output frequency and laser output intensity is shown in fig. 4.
Secondly, the gas chamber absorption module simulates the laser (namely S) modulated by the light source control module 1 ) After the absorption process of the methane gas absorption cell (the simulation environment is set to normal temperature and pressure, a lorentz linear function is selected for simulation, the parameters of the gas chamber are set to the absorption path length L =50cm, the ambient temperature 296K and the pressure 1 atm), specifically, the transmission light intensity signal (namely, S) of the incident laser absorbed by the methane gas 2 ) See figure 5 for a variation diagram.
A noise module which is normal random distribution noise added in the second harmonic signal acquisition system, simulates white noise generated by a photoelectric detector, and superposes a transmission light intensity signal (namely S) after noise on a transmission light intensity signal output by the gas chamber absorption module 3 ) The transmitted light intensity signal (i.e. S) output after the noise is superimposed by the noise module 3 ) See figure 6 for a schematic of the variation.
And the signal processing module is used for performing denoising and filtering processing on the signal to be detected by using a digital lock-in amplifier through a phase-sensitive detection technology (namely multiplying the transmitted light intensity signal by a double-frequency reference signal with the phase difference of 90 degrees by using the lock-in amplifier principle, and then filtering out a direct current component by using a low-pass filter to obtain a second harmonic signal of the transmitted light intensity signal). A schematic diagram of the second harmonic signal output by the signal processing module (i.e., output by the second harmonic signal acquisition system) is shown in fig. 7.
And S3, introducing the second harmonic signals extracted in the step S2 into MATLAB, and acquiring the time domain peak value mean values of the second harmonic signals of methane gas at different concentrations through an MATLAB algorithm to obtain time domain characteristic points. The time domain characteristic points of the second harmonic signal are schematically shown in fig. 8.
MATLAB is a commercial mathematical software, meaning a matrix factory (matrix laboratory), integrating a plurality of powerful functions such as numerical analysis, matrix calculation, scientific data visualization, modeling and simulation of nonlinear dynamic systems and the like into an easy-to-use window environment, and is called three math software together with Mathemica and Maple.
And S4, decomposing the second harmonic signals extracted in the step S2 through discrete wavelet transform, and performing fast Fourier transform on the decomposed wavelet coefficients to obtain corresponding frequency spectrums. On the premise of knowing the change of the concentration value of the methane gas, acquiring second harmonic signal frequency component information related to the change of the concentration value of the methane gas through a large amount of experimental data; as shown in fig. 9a, 9b, and 9c, schematic diagrams of detail components from layer d1 to layer d14 corresponding to discrete wavelet transform are shown, and meanwhile, as shown in fig. 9c, frequency component amplitude information obtained based on detail components of layer d11 after discrete wavelet transform is related to methane concentration value variation through experimental study; frequency component amplitude points are acquired through an MATLAB algorithm and are recorded as frequency domain characteristic points, and a schematic diagram of the frequency domain characteristic points of the second harmonic signals is shown in FIG. 10.
And S5, performing correlation analysis, and comparing and analyzing the time domain characteristic points and the frequency domain characteristic points obtained in the steps S3 and S4 with the change trend of the corresponding methane gas concentration values respectively to find that the time domain characteristic points and the frequency domain characteristic points are in positive correlation with each other in height and the Pearson coefficient is close to 1.
S6, carrying out Simulink experimental analysis, changing the concentration value of the methane gas only with the other system parameters unchanged, taking the concentration of 200ppm as a variable (for example, selecting methane concentrations of 200ppm, 400ppm, 600ppm, 800ppm, \8230; and the like in sequence), repeating the steps S2-S4, collecting 30 groups of time domain characteristic points and frequency domain characteristic points of second harmonic signals of the methane gas under different concentrations, and storing data;
and S7, dividing the 30 groups of data into a correction set (20 groups) and a test set (10 groups), taking the time domain characteristic points and the frequency domain characteristic points as independent variables, taking the methane concentration value as a dependent variable, and establishing a gas concentration inversion model for the two groups of data by utilizing partial least squares regression.
Specifically, 20 sets of data were used as an orthogonal set to build a gas concentration inversion model, and 10 sets of data were used as a test set to examine the built model.
And S8, collecting a second harmonic signal of the gas to be detected by adopting a designed second harmonic signal collection system, obtaining a time domain characteristic point and a frequency domain characteristic point of the gas to be detected, inputting the time domain characteristic point and the frequency domain characteristic point into the gas concentration inversion model in the step S7, and predicting the concentration of the gas to be detected.
And S9, carrying out error analysis on the concentration value of the gas to be detected predicted in the step S8 by adopting an RMSEP method. Namely, after the gas concentration inversion model prediction, the result shows that the RMSEP value of the test set of the gas concentration detection method based on the time-frequency domain joint analysis is 1.871 10^ -6 (the RMSEP value of the test set of the traditional second harmonic signal time-domain peak value inversion gas concentration method is 9.6533 10^ -6), and the prediction accuracy of the gas concentration prediction method is improved by nearly one order of magnitude.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.
Claims (8)
1. A TDLAS-based time-frequency domain combined gas concentration inversion method is characterized by comprising the following steps of:
step 1, setting the gas concentration of standard gas to be A 1 Acquiring a second harmonic signal of the standard gas based on a TDLAS technology;
step 2, extracting A 1 Obtaining a time domain characteristic point by a second harmonic signal peak value of the concentration lower standard gas;
step 3, for A 1 Decomposing a second harmonic signal of the concentration lower standard gas, and obtaining a corresponding frequency spectrum through fast Fourier transform;
step 4, extracting A 1 Obtaining a frequency domain characteristic point by using a second harmonic frequency amplitude component of the concentration lower standard gas;
step 5,Repeating the steps 1 to 4, and changing the concentration value of the standard gas to A 2 、……、A n N is an integer greater than 2, and the other parameters are unchanged, so that time domain characteristic points and frequency domain characteristic points of second harmonic signals of a plurality of concentration index gases are obtained, and data are stored;
step 6, establishing a gas concentration inversion model by taking the gas concentration value of the standard gas as a dependent variable and taking the time domain characteristic point and the frequency domain characteristic point corresponding to the standard gas concentration as independent variables;
and 7, collecting a second harmonic signal of the gas to be detected, obtaining time domain characteristic points and frequency domain characteristic points of the gas to be detected, and predicting the concentration of the gas to be detected by adopting a gas concentration inversion model.
2. The TDLAS-based time-frequency domain combined gas concentration inversion method as claimed in claim 1, wherein: the acquisition of the second harmonic signals of the standard gas and the gas to be detected is realized by a designed second harmonic signal acquisition system;
acquiring a second harmonic signal, comprising the following steps:
step 101, the initial laser intensity S is modulated by the wavelength modulation technique 0 Modulating and outputting modulated laser light S 1 ；
Step 102, inputting modulated laser S 1 And outputs a transmitted light intensity signal S after being absorbed by the standard gas or the gas to be measured 2 ；
Step 103, for the transmitted light intensity signal S 2 Performing superposition noise processing to output transmitted light intensity signal S 3 ；
104, utilizing the phase-locked amplification technique to the transmitted light intensity signal S 3 And processing and outputting a second harmonic signal.
3. The TDLAS-based time-frequency domain combined gas concentration inversion method as claimed in claim 1 or 2, wherein: the extraction methods of the second harmonic signal peak value and the second harmonic frequency amplitude component are both MATLAB algorithm.
4. The TDLAS-based time-frequency domain combined gas concentration inversion method as claimed in claim 1 or 2, wherein: in step 3, the second harmonic signal is decomposed by a discrete wavelet transform method.
5. The TDLAS-based time-frequency domain combined gas concentration inversion method as claimed in claim 1 or 2, wherein: in step 6, a gas concentration inversion model is established by a partial least squares regression method.
6. The TDLAS-based time-frequency domain combined gas concentration inversion method as claimed in claim 1 or 2, wherein: the gas concentration inversion method further comprises a step 8 of performing error analysis on the predicted concentration value of the gas to be measured by the RMSEP method.
7. A second harmonic signal acquisition system for acquisition of a second harmonic signal in the method of any one of claims 1 to 6, wherein: the system comprises a light source control module, an air chamber absorption module, a noise module and a data processing module;
the light source control module modulates incident laser and outputs modulated laser, the input of the light source control module is a low-frequency sawtooth wave signal, a high-frequency sine wave signal, initial laser intensity and laser center frequency, and the output of the light source control module is the frequency and light intensity of the modulated laser;
the gas chamber absorbs the modulated laser absorption of the standard gas or the gas to be measured in the module, and outputs the transmitted light intensity signal;
the noise module is used for generating white noise and performing noise superposition processing on the transmitted light intensity signal output by the gas chamber absorption module;
and the data processing module is used for processing the transmission light intensity signal of the superimposed noise and outputting a second harmonic signal of the standard gas or the gas to be detected.
8. A gas inversion apparatus, characterized by: comprising a second harmonic signal acquisition system as claimed in claim 7.
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| EP3139152A1 (en) * | 2015-09-04 | 2017-03-08 | Siemens Aktiengesellschaft | Optical methane detector using higher harmonic background functions for determining the methane concentration |
| WO2018223107A1 (en) * | 2017-06-02 | 2018-12-06 | Nec Laboratories America, Inc. | Wavelength modulation spectroscopy gas sensor calibration |
| CN110907398A (en) * | 2019-12-31 | 2020-03-24 | 厦门大学 | Gas concentration measuring method and measuring device |
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