CN115015113A - Spectrum gas parameter measuring method and device based on signal power spectrum analysis - Google Patents
Spectrum gas parameter measuring method and device based on signal power spectrum analysis Download PDFInfo
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Abstract
The invention relates to a method and a device for measuring spectral gas parameters based on signal power spectrum analysis, wherein the method comprises the following steps: obtaining modulated light; the modulated light is divided into three beams by the optical fiber beam splitter, and one beam passes through the measuring cell to obtain a transmitted light intensity signal containing gas concentration information
A beam of light intensity signal I 0 (t); the other beam obtains an etalon signal I containing a time-frequency relation υ (t); will I υ (t) converting the initial set concentration value X into a spectral absorption signal alpha (upsilon); will I 0 (t) is combined with alpha (upsilon) to obtain a simulated transmitted light intensity signal according to Beer-Lambert law
Analysis by power spectral density analysis algorithmAnalysis of
Obtaining the power ratio R of the second harmonic and the first harmonic in the frequency domain S Further, X and R are represented S R of the relationship between S -an X database; analysis using power spectral density analysis algorithms
Obtaining the power ratio R of the second harmonic and the first harmonic in the frequency domain M R is to be M Substitution into R S -an X database for interpolating measurements X of the gas concentration M . The sensitivity and the precision of gas parameter measurement are improved.
Description
Technical Field
The invention relates to the technical field of gas detection, in particular to a spectral gas parameter measuring method and device based on signal power spectrum analysis.
Background
In the prior art, gas detection related technologies are mainly classified into two types according to the measurement principle: spectroscopic and non-spectroscopic methods. The non-spectroscopic method mainly includes a chemical analysis method and an electrical gas detection method. Chemical analysis methods can separate mixed gases efficiently, but the response time is slow. The electrical gas detection method has high sensitivity and wide detection range, but needs to be in contact with a measurement environment and has poor capability of identifying the gas type. The spectrum method is used for inverting the gas parameter information by measuring the target gas spectrum related parameters, can detect various gases, and has the advantages of non-contact measurement, quick response time and the like. The spectroscopy mainly includes fourier transform infrared absorption spectroscopy (FTIR), tunable semiconductor laser absorption spectroscopy (TDLAS), and the like. FTIR technique is mainly based on Michelson interferometer principle, and infrared source sends the collimated light through collimating lens, is received by telescope system after the gaseous absorption of awaiting measuring, assembles the detector through the interferometer again to obtain the gaseous interference signal of awaiting measuring, can obtain gaseous absorption spectrum information under the different concentration after Fourier transform, thereby calculate gaseous concentration. FTIR devices are bulky, relatively slow in response, and relatively expensive. The TDLAS technology is a spectral measurement method based on the narrow linewidth characteristic of a semiconductor DFB laser, can realize simultaneous measurement of multiple components and multiple parameters of mixed gas, has very strong universality and high measurement resolution, and can measure the concentration of trace gas by selecting a proper characteristic absorption spectral line of gas to be measured. Among them, the measurement technique of Wavelength Modulation Spectroscopy (WMS) based on TDLAS has the advantages of non-intrusive, non-interfering measurement object, fast response time, strong anti-electromagnetic interference capability, small occupied space of measurement instrument equipment system, easy installation, etc., and has been widely used for monitoring combustion field temperature and component concentration. In order to accurately obtain a calibration-free target spectrum, a few scholars have tried processing methods such as 1f normalization, fourier transform-based multi-parameter harmonic fitting, absorption line type correction, center wavelength drift correction and the like. Among them, the conventional 2f/1f method without calibration is most commonly used because of its advantages of easy operation, no need of complex analysis model, suitability for various measurement environments, etc.
However, in the detection of gas parameters in a complex environment, due to the influence of disturbance factors such as turbulence and strong vibration in the environment, signals obtained by measurement by the conventional WMS method are distorted, and a secondary signal extracted by a common harmonic extraction method has a large error even exceeds a limited range when relevant information of the gas parameters is inverted. When mechanical vibration, light beam jitter and window pollution exist in a measurement environment, a measured normalized second harmonic signal is easily polluted by noise, so that the measurement sensitivity and precision are reduced. To enhance detection sensitivity while eliminating noise, data averaging is often used in calibration-free absorption spectroscopy. However, analysis shows that the environmental noise not only causes the fluctuation of the harmonic wave, but also causes the spurious enhancement of the harmonic wave. As a result, averaging the measurement data only improves the measurement accuracy, but does not contribute to the systematic error caused by noise.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a spectral gas parameter measuring method and device based on signal power spectrum analysis, aiming at enhancing the detection sensitivity and the detection precision of gas parameters of a calibration-free wavelength modulation-based gas parameter measuring system and improving the anti-noise performance of the measuring system.
The technical scheme adopted by the invention is as follows:
one aspect of the present application provides a method for measuring a spectral gas parameter based on signal power spectrum analysis, including the following steps:
(1) the function generator will scan at frequency f s Has a signal superposition modulation frequency of f m The signal of (2) is input into a laser controller, and the laser controller tunes the output wavelength and light intensity of the DFB laser;
(2) dividing the modulated light obtained in the step (1) into three light signals through an optical fiber beam splitter, wherein one light signal is emitted from the emitting end I of a laser collimator, passes through a measuring cell and is received by a photoelectric detector I to obtain a transmitted light intensity signal I containing gas concentration information t M (t); one beam is emitted by the emitting end II of the laser collimator and is directly received by the photoelectric detector II to obtain a background light intensity signal I without absorption information 0 (t); the other beam is emitted by a laser collimator emitting end III, is received by a photoelectric detector III after passing through an optical etalon to obtain an etalon signal I containing a time-frequency relation υ (t)；
(3) Coupling the etalon signal I υ (t) converting by combining a HITRAN database, an initial set concentration value X and other known parameters to obtain a spectral absorption rate signal alpha (upsilon) based on wavelength;
(4) the background light intensity signal I 0 (t) is combined with the spectral absorption rate signal alpha (upsilon) to obtain a simulated transmitted light intensity signal containing gas related information according to the Beer-Lambert law
(5) For the simulation transmitted light intensity signal
Analyzing by adopting a power spectral density analysis algorithm to obtain
Power ratio R of second harmonic to first harmonic in frequency domain S Thereby obtaining R corresponding to different initial setting concentration values X S Establishing the representations X and R S Simulated R of the relationship between S -an X database;
(6) the transmitted light intensity signal I obtained by the measurement in the step (2) t M (t) similarly analyzing by using a power spectral density analysis algorithm to obtain I t M (t) power ratio R of second harmonic to first harmonic in frequency domain M R is to be M Substituting R obtained in the step (5) S An X database for obtaining by interpolation a measured value X of the gas concentration M 。
The further technical scheme is as follows:
in the formula, P 2f 、P 1f Respectively a second harmonic and a first harmonic f Amplitude of transmitted light intensity after Fourier transform, f m Is the modulation frequency, f c1 、f c2 Cut-off frequencies of the first harmonic and the second harmonic respectively; lambda [ alpha ] 1 、λ 2 The average values of the transmitted light intensity power spectral densities in the non-absorption frequency ranges near the first harmonic and the second harmonic are respectively.
λ 1 Can be calculated by the following formula:
λ 2 can be calculated by the following formula:
the device comprises a function generator, a laser controller, a DFB laser, an optical fiber beam splitter, three laser collimator transmitting ends, a measuring pool filled with gas to be measured, an optical etalon, three photoelectric detectors, a data acquisition card and a computer;
the function generator inputs the scanning signal superposed with the modulation signal into the laser controller through an internal channel of the function generator;
the laser controller simultaneously tunes the output wavelength and the light intensity of the DFB laser;
laser emitted by the DFB laser is divided into three light signals through the two optical fiber beam splitters, one light signal is emitted from the emitting end I of the laser collimator, passes through the measuring cell, is received by the photoelectric detector I and is converted into an electric signal, and a transmitted light intensity signal I containing gas concentration information is obtained t M (t)；
One beam is emitted by the emitting end II of the laser collimator and is directly received by the photoelectric detector II to obtain a background light intensity signal I without absorption information 0 (t)；
The other beam is emitted by the emitting end III of the laser collimator, passes through the optical etalon 7 and is received by the photoelectric detector III to obtain an etalon signal I containing a time-frequency relation υ (t)；
The three signals are collected by a data acquisition card and input into a computer for real-time analysis or storage so as to perform other analysis and processing of subsequent signals;
still include signal analysis and processing module, signal analysis and processing module is used for:
coupling the etalon signal I υ (t) converting by combining a HITRAN database, an initial set concentration value X and other known parameters to obtain a spectral absorption rate signal alpha (upsilon) based on wavelength;
the background light intensity signal I 0 (t) is combined with the spectral absorption rate signal alpha (upsilon) to obtain a simulated transmitted light intensity signal containing gas related information according to the Beer-Lambert law
For the simulation transmitted light intensity signal
Analyzing by adopting a power spectral density analysis algorithm to obtain
Power ratio R of second harmonic to first harmonic in frequency domain S Establishing R corresponding to different initial set concentration values X S Obtaining simulated R S -an X database;
for the transmitted light intensity signal
The power spectral density analysis algorithm is also adopted for analysis to obtain
Power ratio R of second harmonic to first harmonic in frequency domain M R is to be M Substituting into the R S An X database for obtaining by interpolation a measured value X of the gas concentration M 。
The invention has the following beneficial effects:
compared with the traditional calibration-free wavelength modulation spectrum technology based on the first harmonic signal and the second harmonic signal, the method provided by the invention can calculate the concentration of the gas by interpolation in a pre-simulated database only by acquiring the power ratio of the second harmonic frequency and the fundamental frequency of the measurement signal in a frequency domain through a power spectrum analysis algorithm, so that the sensitivity and the precision of gas parameter measurement are improved; meanwhile, due to the sparse characteristic of the power spectrum of the modulation signal, the invention can eliminate the influence of noise on the measurement result to a certain extent, and expand the application range of the spectral absorption method; the method has the characteristics of good applicability, wide application scene and the like, so the method can be used for the diagnosis and measurement of flame temperature and component concentration in complex environments such as an aerospace engine combustion chamber and the like.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.
Drawings
FIG. 1 is a schematic flow diagram of the process of the present invention.
Fig. 2 is a measurement result obtained by continuously measuring methane having a concentration of 1.10% by using the conventional method and the method of the present invention.
Fig. 3 is an analysis of the results of the alan variance over integration time using both conventional and inventive methods.
FIG. 4 is a schematic view of the structure of the device of the present invention.
Detailed Description
The following describes embodiments of the present invention with reference to the drawings.
The combustion chamber is an important part of the aircraft engine, and releases chemical energy in fuel through combustion, so that the chemical energy is converted into heat energy, high-temperature and high-pressure fuel gas is generated, and the working capacity of the engine is improved. When designing the combustion chamber, need to guarantee that combustion stability is good, combustion efficiency is high, emission pollution is few etc. combustion characteristic. The combustion products of an aircraft engine combustion chamber are predominantly residual air (O) 2 And N 2 ) Carbon oxides (CO and CO) 2 ) Nitrogen oxides (NO and NO) 2 ) Gases such as unburned hydrocarbon compounds (TCH) and solid fine particles. The characteristic gas is an important indicator gas reflecting the combustion characteristics. Its presence indicates that the fuel is not completely combusted and combustion efficiency is to be improved. The detection of the characteristic gas parameters provides important data for optimizing the combustion efficiency, obtaining the design of a combustion chamber and selecting the fuel equivalence ratio. In this regard, the present application provides a method for measuring spectroscopic gas parameters based on signal power spectral analysis, see fig. 1, comprising the steps of:
(1) the function generator will scan at frequency f s Has a signal superposition modulation frequency of f m The signal of (2) is input into a laser controller, and the laser controller tunes the output wavelength and light intensity of the DFB laser;
(2) dividing the modulated light obtained in the step (1) into three light signals through an optical fiber beam splitter, wherein one light signal is emitted from the emitting end I of a laser collimator, passes through a measuring cell and is received by a photoelectric detector I to obtain a transmitted light intensity signal I containing gas concentration information t M (t); one beam is emitted by the emitting end II of the laser collimator and is directly received by the photoelectric detector II to obtain a background light intensity signal I without absorption information 0 (t); the other beam is collimated by laserThe etalon signal I containing the time-frequency relation is obtained by being sent by the transmitting end III of the straight device and received by the photoelectric detector III after passing through the optical etalon υ (t)；
(3) For the etalon signal I υ (t) carrying out peak searching fitting processing to obtain a wave number signal v (t) changing along with time, and then converting by combining information in a HITRAN database, an initial set concentration value X and other known parameters to obtain a spectral absorption rate signal alpha (upsilon) based on wave number according to the following formula:
wherein P, T, L, S (T), v 0 Respectively representing ambient pressure, temperature, measurement area length, line intensity and spectral line center wave number; t is the temperature;
as a linear function of the absorption spectrum,. DELTA.v C For collisional broadening, Δ v D Is doppler broadening.
(4) The background light intensity signal I 0 (t) is combined with the spectral absorption rate signal alpha (upsilon) to obtain a simulated transmitted light intensity signal containing gas related information according to the Beer-Lambert law as follows
(5) For the simulation transmitted light intensity signal
Analyzing by adopting a power spectral density analysis algorithm (PSD) to obtain
Power ratio R of second harmonic to first harmonic in frequency domain S Thereby obtaining R corresponding to different initial setting concentration value X S The form R can be obtained by fitting S kX + b denotes X and R S Simulated R of the relationship between S -an X database;
in the formula, P 2f 、P 1f Respectively a second harmonic and a first harmonic, a f And b f Amplitude, f, of transmitted light intensity and noise after Fourier transform m Is the modulation frequency, f c1 And f c2 Cut-off frequencies of the first harmonic and the second harmonic respectively;
the noise term in the equation can be obtained in the frequency range of the power spectral density of the transmitted light intensity that is not related to absorption. Thus, the above formula can be converted to the following formula:
in the formula, λ 1 、λ 2 The average values of the transmitted light intensity power spectral densities in the non-absorption frequency ranges near the first harmonic and the second harmonic are respectively.
λ 1 Can be calculated by the following formula:
λ 2 can be calculated by the following formula:
(6) the transmitted light intensity signal obtained by the measurement in the step (2)
The power spectral density analysis algorithm is also adopted for analysis to obtain
Power ratio R of second harmonic to first harmonic in frequency domain M R is to be M Substituting R obtained in the step (5) as an ordinate S X and R in kX + b form S A database of relationships between the gas concentration and the gas concentration, and obtaining the abscissa, i.e. the measured value X of the gas concentration by interpolation M 。
As shown in FIG. 2, the results of measurement of methane having a concentration of 1.10% were obtained by continuously measuring the conventional 2f/1f method and the method of this example.
The measurement conditions are the same and are all under the influence of noise in the actual measurement environment, as can be seen from fig. 2, the final measurement values of the two measurement methods have certain fluctuation, but it is obvious that the measurement result of the PSD analysis method provided in this embodiment has smaller error and smaller fluctuation compared with the conventional method, and the detection sensitivity of the gas parameter measurement system and the detection accuracy of the gas parameters are increased.
As shown in fig. 3, the results of the analysis of the Allan variance with the integration time of the measurement results by the conventional method and the method of the present embodiment are shown. It can be seen that the PSD analysis method of the present embodiment always performs better than the conventional 2f/1f method in the integration time range of 1s to 60 s. As can be seen from the Allan variance plot, when the integration time is 1s, the CH of the conventional 2f/1f method and the PSD analysis method of the embodiment 4 The gas detection limits were 31.34ppmv, 11.85ppmv, respectively; when the integration time is 60s, the conventional 2f/1f method and the CH of the PSD analysis method of the embodiment 4 The gas detection limit was reduced to 8.20ppmv, 6.79ppmv, respectively. Namely, the method provided by the application has a lower detection lower limit.
The application also provides a spectral gas parameter measuring device based on signal power spectrum analysis, which is shown in figure 4 and comprises a function generator 1, a laser controller 2, a DFB laser 3, an optical fiber beam splitter 4, three laser collimator transmitting ends 5, a measuring pool 6 filled with gas to be measured, an optical etalon 7, three photoelectric detectors 8, a data acquisition card 9 and a computer 10;
the function generator 1 inputs the scanning signal superposed with the modulation signal into the laser controller 2 through an internal channel thereof;
the laser controller 2 tunes the output wavelength and light intensity of the DFB laser 3 at the same time;
laser emitted by the DFB laser 3 is divided into three light signals by the two optical fiber beam splitters 4, one light signal is emitted by the emitting end I of the laser collimator, passes through the measuring cell, is received by the photoelectric detector I and is converted into an electric signal, and a transmitted light intensity signal I containing gas concentration information is obtained t M (t)；
One beam is emitted by the emitting end II of the laser collimator and is directly received by the photoelectric detector II to obtain a background light intensity signal I without absorption information 0 (t)；
The other beam is emitted by the emitting end III of the laser collimator, passes through the optical etalon 7 and is received by the photoelectric detector III to obtain an etalon signal I containing a time-frequency relation υ (t)；
The three paths of signals are collected by a data acquisition card 9 and input into a computer 10 for real-time analysis or storage so as to perform other analysis and processing of subsequent signals;
still include signal analysis and processing module, signal analysis and processing module is used for:
etalon signal I υ (t) converting by combining a HITRAN database, an initial set concentration value X and other known parameters to obtain a spectral absorption rate signal alpha (upsilon) based on wavelength;
the background light intensity signal I 0 (t) is combined with the spectral absorptivity signal alpha (upsilon), and a simulated transmitted light intensity signal containing gas related information is obtained according to the Beer-Lambert law
For simulation transmitted lightStrong signal
Analyzing by adopting a power spectral density analysis algorithm to obtain
Power ratio R of second harmonic to first harmonic in frequency domain S Establishing R corresponding to different initial setting concentration values X S Obtaining a simulated R S -an X database;
for transmitted light intensity signal
The power spectral density analysis algorithm is also adopted for analysis to obtain
Power ratio R of second harmonic to first harmonic in frequency domain M R is to be M Substitution into R S An X database for obtaining by interpolation a measured value X of the gas concentration M 。
Among them, the DFB laser 3, i.e., the distributed feedback laser, can continuously emit stable laser light according to a set condition, and its characteristic output wavelength needs to be determined according to a gas to be measured.
Those of ordinary skill in the art will understand that: although the present invention has been described in detail with reference to the foregoing embodiments, it will be apparent to those skilled in the art that changes may be made in the embodiments and/or equivalents thereof without departing from the spirit and scope of the invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (4)
1. A spectral gas parameter measurement method based on signal power spectrum analysis is characterized by comprising the following steps:
(1) letter boxThe number generator will scan at frequency f s Has a signal superposition modulation frequency of f m The signal of (2) is input into a laser controller, and the laser controller tunes the output wavelength and light intensity of the DFB laser;
(2) dividing the modulated light obtained in the step (1) into three light signals through an optical fiber beam splitter, wherein one light signal is emitted from the emitting end I of a laser collimator, passes through a measuring cell and is received by a photoelectric detector I to obtain a transmitted light intensity signal I containing gas concentration information t M (t); one beam is emitted by the emitting end II of the laser collimator and is directly received by the photoelectric detector II to obtain a background light intensity signal I without absorption information 0 (t); the other beam is emitted by a laser collimator emitting end III, is received by a photoelectric detector III after passing through an optical etalon to obtain an etalon signal I containing a time-frequency relation υ (t)；
(3) Coupling the etalon signal I υ (t) converting by combining a HITRAN database, an initial set concentration value X and other known parameters to obtain a spectral absorption rate signal alpha (upsilon) based on wavelength;
(4) the background light intensity signal I 0 (t) is combined with the spectral absorption rate signal alpha (upsilon) to obtain a simulated transmitted light intensity signal containing gas related information according to the Beer-Lambert law
(5) For the simulation transmitted light intensity signal
Analyzing by adopting a power spectral density analysis algorithm to obtain
Power ratio R of second harmonic to first harmonic in frequency domain S Thereby obtaining R corresponding to different initial setting concentration values X S Establishing the representations X and R S Simulated R of the relationship between S -an X database;
(6) the transmitted light intensity signal obtained by the measurement in the step (2)I t M (t) similarly analyzing by using a power spectral density analysis algorithm to obtain I t M (t) power ratio R of second harmonic to first harmonic in frequency domain M R is to be M Substituting R obtained in the step (5) S An X database for obtaining by interpolation a measured value X of the gas concentration M 。
2. The method of claim 1, wherein the spectral gas parameter measurement method based on signal power spectral analysis,
power ratio R of second harmonic to first harmonic in frequency domain S Calculated from the following formula:
in the formula, P 2f 、P 1f Respectively a second harmonic and a first harmonic, a f Amplitude of transmitted light intensity after Fourier transform, f m Is the modulation frequency, f c1 、f c2 Cut-off frequencies of the first harmonic and the second harmonic respectively; lambda [ alpha ] 1 、λ 2 The average values of the transmitted light intensity power spectral densities in the non-absorption frequency ranges near the first harmonic and the second harmonic are respectively.
3. The method of claim 2, wherein the spectral gas parameter measurement method based on signal power spectral analysis,
λ 1 can be calculated by the following formula:
λ 2 can be calculated by the following formula:
4. a spectral gas parameter measuring device based on signal power spectrum analysis is characterized by comprising a function generator, a laser controller, a DFB laser, an optical fiber beam splitter, three laser collimator transmitting ends, a measuring cell filled with gas to be measured, an optical etalon, three photoelectric detectors, a data acquisition card and a computer;
the function generator inputs the scanning signal superposed with the modulation signal into the laser controller through an internal channel of the function generator;
the laser controller simultaneously tunes the output wavelength and the light intensity of the DFB laser;
laser emitted by the DFB laser is divided into three light signals by the two optical fiber beam splitters, one light signal is emitted by the emitting end I of the laser collimator, passes through the measuring cell, is received by the photoelectric detector I and is converted into an electric signal, and a transmitted light intensity signal I containing gas concentration information is obtained t M (t)；
One beam is emitted by the emitting end II of the laser collimator and is directly received by the photoelectric detector II to obtain a background light intensity signal I without absorption information 0 (t)；
The other beam is emitted by the emitting end III of the laser collimator, passes through the optical etalon 7 and is received by the photoelectric detector III to obtain an etalon signal I containing a time-frequency relation υ (t)；
The three signals are collected by a data acquisition card and input into a computer for real-time analysis or storage so as to perform other analysis and processing of subsequent signals;
still include signal analysis and processing module, signal analysis and processing module is used for:
coupling the etalon signal I υ (t) converting by combining a HITRAN database, an initial set concentration value X and other known parameters to obtain a spectral absorption rate signal alpha (upsilon) based on wavelength;
the background light intensity signal I 0 (t) is combined with the spectral absorbance signal α (upsilon) in dependence onObtaining the simulated transmitted light intensity signal containing the gas related information according to the Beer-Lambert law
For the simulation transmitted light intensity signal
Analyzing by adopting a power spectral density analysis algorithm to obtain
Power ratio R of second harmonic to first harmonic in frequency domain S Establishing R corresponding to different initial setting concentration values X S Obtaining a simulated R S -an X database;
for the transmitted light intensity signal I t M (t) similarly analyzing by using a power spectral density analysis algorithm to obtain I t M (t) power ratio R of second harmonic to first harmonic in frequency domain M A 1 to R M Substituting into the R S An X database for obtaining by interpolation a measured value X of the gas concentration M 。
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