CN109100325A - A kind of gas concentration measuring method based on spectral absorption second harmonic feature extraction - Google Patents
A kind of gas concentration measuring method based on spectral absorption second harmonic feature extraction Download PDFInfo
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- G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
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Abstract
The invention discloses a kind of gas concentration measuring methods based on spectral absorption second harmonic feature extraction, this method is suitable for spectral absorption second harmonic peak height-peak width characteristic relation of Kao Lv Fu Yite line style under any index of modulation by establishing, it realizes and calculates integrated absorbance using harmonic characteristic, to obtain gas parameter information；Logarithm process technology is used in this method implementation process simultaneously, the influence that residual amplitude modulation distorts to harmonic signal is directly eliminated, improves measurement accuracy；Measurement method of the present invention reduces the requirement to hardware system without carrying out complicated least-squares iteration the Fitting Calculation and only needing once to be filtered；Wave character point information still can be more accurately extracted when complete harmonic signal can not be obtained, measurement lower limit is lower；Without using from excessive parameters such as ceofficient of spread, the collision broadening coefficient of various other components, temperature-independent indexes, the dependence to database parameter is reduced in database, application range is wider.
Description
Technical field
The present invention relates to a kind of gas concentration measuring methods based on spectral absorption second harmonic feature extraction, belong to sharp
Optical absorption spectra technical field.
Background technique
Tunable diode laser absorption spectroscopy techniques (tunable diode laser absorption
Spectroscopy, TDLAS) because the multi-parameters on-line measurement such as concentration of component, temperature, pressure and speed may be implemented,
And has the characteristics that highly sensitive, Fast Time Response and non-contact, have in detection and the combustion diagnosis field of trace gas
Wide application prospect.
In order to reduce influence of the noise to measurement result, signal-to-noise ratio is improved, frequently with wavelength tune on signal detecting method
Spectrum (WMS) method processed, wherein second-harmonic detection is the most commonly used.Frequency-modulated spectroscopy is when actual quantification measures, measurement
As a result it generally requires to demarcate by calibrating gas, however due to the component of under test gas in actual field environment and calibrating gas
Component is different, and may change at any time, and the gas concentration measurement obtained completely according to calibration mode can have certain mistake
Difference.In order to solve this problem, researcher has developed a series of non-calibrating WMS methods, and main includes being based on Hitran number
According to the harmonic analysis method of library emulation and WMS-2f/1f waveform fitting etc..However the harmonic wave point based on the emulation of Hitran database
Analysis method needs accurate line parameters (for example, collision broadening coefficient, the temperature-independent from ceofficient of spread, various other components refer to
Number etc.) and laser modulation characterisitic parameter, measurement result be affected by line parameters.WMS-2f/1f waveform fitting method
It without more line parameters, is widely used in the measurement of gas parameter in recent years, however this method needs to carry out greatly
The iterative fitting of amount calculates, and needs to carry out the filtering processing of multiple harmonic, and calculation amount is larger, to hardware and fitting algorithm requirement
It is higher.More seriously when absorption is weaker or measurement environment is more severe, influenced by background variation and residual amplitude modulation,
Harmonic wave form signal is obtained it is difficult to extract complete, WMS-2f/1f waveform fitting method can not be applied at this time.On the other hand, due to humorous
The information being richly stored in wave signal line style about spectral absorption also can extract light from the wave character of harmonic signal
Spectrum absorptivity is to realize the measurement of gas parameter.However existing waveshape feature abstraction spectral absorption method is often only applicable in
Frequency analysis under the small index of modulation or lorentzian curve, measurement noise is relatively low, and application range is limited.Therefore, develop
A kind of signal-to-noise ratio is high, and independent of spectral line data library parameter, and the non-calibrating WMS method being suitable under any index of modulation is especially
It is important.
Summary of the invention
Technical problem to be solved by the invention is to provide a kind of gas based on spectral absorption second harmonic feature extraction
Bulk concentration measurement method, the gas concentration measuring method signal-to-noise ratio is high, independent of spectral line data library parameter, and is suitable for any
The gas concentration measurement of non-calibrating Wavelength modulation spectroscopy under the index of modulation.
In order to solve the above technical problems, the technical scheme adopted by the invention is as follows:
A kind of gas concentration measuring method based on spectral absorption second harmonic feature extraction, the measurement method include with
Lower step:
Step 1, laser modulates optical maser wavelength, photodetector while scanning absorption line with high frequency sinusoidal signal
The background light intensity signal I of no absorption is measured respectively0With the transmitted light intensity signal I for having absorptiont；
Step 2, interference peak-to-peak signal of the measurement laser after solid etalon, and freely composing according to the solid etalon
Time domain light intensity signal is converted into frequency domain light intensity signal by spacing FSR, to obtain the variation relation V of laser scanning relative wavenumbers
(t) and the size a of modulation depth；
Step 3, transmitted light intensity signal I measurement obtainedtWith background light intensity signal I0Logarithm process is carried out, spectrum is obtained
Absorptivity signal alpha (v)；
Step 4, locking phase filtering processing is carried out to spectral absorption signal alpha (v), obtains its corresponding second harmonic signal Hα；
Step 5, second harmonic signal H is calculated using peak-seeking algorithmαCenter peak heightsCombining laser scanning phase
Second harmonic signal H is calculated to the variation relation V (t) of wave numberαIn the secondary lobe width Λ of frequency domain；
Step 6, it establishes under any index of modulation, the corresponding second harmonic signal H of spectral absorption α (v)αCenter peak height
DegreeAnd secondary lobe width Λ and integrated absorbance A and absorption line Lorentz broadening λLRelationship；
Step 7, absorption line Lorentz broadening λ is initializedL=λL0, it is absorbed spectral line according to known temperature computation
Gauss broadens λG, in conjunction with the modulation depth a that measurement obtains, theoretical secondary lobe width Λ is calculated0；
Step 8, the theoretical secondary lobe width Λ that the secondary lobe width Λ and step 7 that judgment step 5 obtains are obtained0Whether meet with
The lower condition of convergence:
In formula, s is preset convergence threshold；
If satisfied, the Lorentz broadening λ for the spectral line that is then absorbedL=λL0, the harmonic wave center peak heights that are obtained in conjunction with step 5Integrated absorbance A is calculated, integrated absorbance A calculation formula is as follows:
If not satisfied, then updating λL0=λnew, return step 7；Wherein, λnewIt is the new of used optimization algorithm generation
Value, optimization algorithm can be randomly selected from algorithmic function each in MATLAB, and the function selected can generate a random new value；
Step 9, the concentration value of gas, the calculating of gas concentration value are calculated according to the integrated absorbance A being calculated
Formula is as follows:
In formula, P is gas stagnation pressure, and S (T) is that the spectral line line under temperature T is strong, and L is to absorb optical length.
Wherein, in step 3, the expression formula of logarithm process are as follows:
α (v)=- ln (It/I0)=A φ (v)
In formula: ItAnd I0Respectively laser transmitted light intensity and laser light incident light intensity；A is integrated absorbance；φ (v) is line style
Function.
Wherein, linear function φ (v) is the convolution of lorentzian curve function and Gaussian lineshape function, using Fu Yite line style
Function description, approximate expression are as follows:
φ (v)=cLφL(v)+cGφG(v)
In formula: φLAnd φGRespectively lorentzian curve function and Gaussian lineshape function；cLAnd cGRespectively Lorentz broadening
λLλ is broadened with GaussGWeight coefficient；v0For laser line centre frequency；λ is the full width at half maximum of absorption line；cL、cGWith λ's
Calculation formula is as follows:
D=(λL-λG)/(λL+λG)
cL=0.6818817+0.6129331d-0.1838439d2-0.1156844d3
cG=0.3246017-0.6182531d+0.1768139d2+0.1210944d3
Second harmonic central peak height and secondary lobe small peak are extracted in the width of frequency domain as characteristic quantity, and establishes and is suitable for
Under any index of modulation, the corresponding second harmonic signal H of spectral absorption α (v)αCenter peak heights and secondary lobe width Λ with
Integrated absorbance A and absorption line Lorentz broadening λLRelationship；
Wherein, the corresponding second harmonic signal H of spectral absorption α (v)αCenter peak heightsWith integrated absorbance A and
Absorption line Lorentz broadening λLRelational expression are as follows:
In formula, I1For 1 rank modified Bessel function of the first kind, parameter m, cL、cGCalculation formula it is as follows:
D=(λL-λG)/(λL+λG)
cL=0.6818817+0.6129331d-0.1838439d2-0.1156844d3
cG=0.3246017-0.6182531d+0.1768139d2+0.1210944d3
M=2a/ λ；
Wherein, the corresponding second harmonic signal H of spectral absorption α (v)αSecondary lobe width Λ and integrated absorbance A and suction
Receive spectral line Lorentz broadening λLRelational expression are as follows:
In formula, r=cG/cL, p1=1.966194179, p2=0.390933340, p3=3.093996758, p4=
0.388999538, p5=0.806546476, p6=-0.289132222, p7=-1.775513203, p8=0.017671096,
p9=1.041510614, p10=-0.146221523, p11=-1.073806845.
Wherein, in step 9, the strong S of spectral line line (T) is varied with temperature and is indicated using following formula:
In formula, h is Planck constant, and h is the light velocity in vacuum, and k is Boltzmann constant, and E is low-level energy, T0=
296K is reference temperature, v0For core frequency, the calculation method of partition function value when Q (T) is temperature T, E and Q (T) is logical
Inquiry HITRAN2016 database is crossed to obtain.
The utility model has the advantages that the spectral absorption that this method is suitable for the consideration Voigt line style under any index of modulation is secondary
Harmonic wave peak height-peak width feature extraction, while logarithm process technology is used, measuring signal is handled, has been reached and WMS-2f/
1f the same light intensity normalizes effect, and the result that logarithm process obtains is unrelated with laser intensity modulation parameter, directly eliminates
The influence that residual amplitude modulation distorts to harmonic signal, improves measurement accuracy；Measurement method of the present invention and WMS-2f/1f wave
Shape approximating method is compared, and without carrying out complicated least-squares iteration the Fitting Calculation and only needing once to be filtered, is dropped
The low requirement to hardware system, and wave character point information still can be accurately extracted when complete harmonic signal can not be obtained, it surveys
It is lower to measure lower limit；Without using in database from ceofficient of spread, the collision broadening coefficient of various other components, temperature-independent index
Etc. excessive parameter, reduce the dependence to database parameter, application range is wider.
Detailed description of the invention
Fig. 1 is the flow chart of gas concentration measuring method of the present invention；
The comparison for the harmonic wave that Fig. 2 obtains for the fitting result of WMS-2f/1f waveform fitting method with measurement method of the present invention
Figure；
Fig. 3 is that difference is matched under gas concentration, the concentration measurement figure of two kinds of gas concentration measuring methods；
Fig. 4 is different under gas concentration, and the relative error of two kinds of gas concentration measuring method measurements compares.
Specific embodiment
Technical scheme of the present invention is further explained with reference to the accompanying drawing.
Fig. 1 is the flow chart of measurement method of the present invention, as shown in Figure 1, the present invention is based on spectral absorption second harmonic spies
The gas concentration measuring method extracted is levied, is specifically comprised the following steps:
Step 1, the background light intensity signal I of no absorption is measured respectively0With the transmitted light intensity signal I for having absorptiont:
Laser modulates optical maser wavelength while scanning absorption line with high frequency sinusoidal signal, and photodetector is surveyed respectively
Measure the background light intensity signal I without absorption0With the transmitted light intensity signal I for having absorptiont；
Step 2, measurement laser goes out the relationship between frequency and time of light:
Interference peak-to-peak signal of the laser after solid etalon is measured, and spacing is freely composed according to the solid etalon
Time domain light intensity signal is converted into frequency domain light intensity signal by FSR, thus obtain laser scanning relative wavenumbers variation relation V (t) and
The size a of modulation depth；
Step 3, spectral absorption α (v) is calculated:
The transmitted light intensity signal I that measurement is obtainedtWith background light intensity signal I0Logarithm process is carried out, spectral absorption is obtained
Signal alpha (v)=- ln (It/I0)=A φ (v)；
Step 4, analysis obtains the second harmonic signal H of spectral absorption signalα:
Spectral absorption signal alpha (v) is handled using digital servo-control process, obtains the x-component and y comprising second harmonic signal
Component:
x2f=α (v) cos (4 π fmt) y2f=α (v) sin (4 π fmt)
In formula, fmFor modulating frequency, x2f、y2fIt is the corresponding x-component of spectral absorption signal alpha (v), y-component respectively；
Then low-pass filtered device extracts the second harmonic X-component and Y-component of each signal:
X2f=lowpass filter (x2f) Y2f=lowpass filter (y2f)
The second harmonic of spectral absorption signal alpha (v) is shown below；
Step 5, analysis obtains spectral absorption signal second harmonic signal HαCenter peak heights and secondary lobe width:
Second harmonic signal H is calculated using peak-seeking algorithmαCenter peak heightsCombining laser scanning relative wavenumbers
Variation relation V (t) second harmonic signal H is calculatedαIn the secondary lobe width Λ of frequency domain；
Step 6, it establishes under any index of modulation, the corresponding second harmonic signal H of spectral absorption α (v)αCenter peak height
DegreeAnd secondary lobe width Λ and integrated absorbance A and absorption line Lorentz broadening λLRelationship:
The corresponding second harmonic signal H of spectral absorption α (v)αCenter peak heightsWith integrated absorbance A and absorption spectra
Line Lorentz broadening λLRelationship expression formula are as follows:
In formula, I1For 1 rank modified Bessel function of the first kind, parameter m, cL、cGCalculation formula it is as follows:
D=(λL-λG)/(λL+λG)
cL=0.6818817+0.6129331d-0.1838439d2-0.1156844d3
cG=0.3246017-0.6182531d+0.1768139d2+0.1210944d3
M=2a/ λ
The corresponding second harmonic signal H of spectral absorption α (v)αSecondary lobe width Λ and integrated absorbance A and absorption line
Lorentz broadening λLRelationship expression formula are as follows:
R=c in formulaG/cL, parameter pi(i=1,2 ..., value 11) it is as shown in table 1:
1 parameter p of tableiValue
Step 7, harmonic wave secondary lobe width theoretical value Λ is calculated0:
Firstly the need of initialization absorption line Lorentz broadening λL=λL0；
λ is broadened according to the Gauss that absorption line can be calculated in known temperatureG, calculation formula is as follows:
λG=7.1623 × 10-7v0(T/M)1/2
In formula, M is the molal weight of under test gas；
In conjunction with the modulation depth a that measurement obtains, and utilize the corresponding second harmonic of spectral absorption α (v) of step 6 foundation
Signal HαSecondary lobe width Λ and integrated absorbance A and absorption line Lorentz broadening λLRelationship theoretical harmonic wave can be calculated
Secondary lobe width Λ0；
Step 8, the theoretical secondary lobe width Λ that the secondary lobe width Λ and step 7 that judgment step 5 obtains are obtained0Whether meet with
The lower condition of convergence:
In formula, ε is preset convergence threshold；
If satisfied, the Lorentz broadening λ for the spectral line that is then absorbedL=λL0, the harmonic wave center peak heights that are obtained in conjunction with step 5Integrated absorbance A can be calculated, integrated absorbance A calculation formula is as follows:
If not satisfied, then updating λL0=λnew, return step 7；Wherein, λnewIt is the new of used optimization algorithm generation
Value, optimization algorithm can arbitrarily be chosen from algorithmic function each in MATLAB, and the function selected can generate a random new value.
Step 9, gas concentration value is calculated:
It can be calculated as follows to obtain gas concentration value according to integrated absorbance A:
In formula, P is gas stagnation pressure, and S (T) is that the spectral line line under temperature T is strong, and L is to absorb optical length.
Absorption line strengths S (T) variation with temperature can indicate are as follows:
In formula, h is Planck constant, and h is the light velocity in vacuum, and k is Boltzmann constant, and E is low-level energy, T0=
296K is reference temperature, v0For core frequency, the calculation method of partition function value when Q (T) is temperature T, E and Q (T) can
To be found in HITRAN2016 database.
Embodiment 1
Below with CH4Molecule 6046.95cm-4Spectral line carries out CH4The measurement of volumetric concentration, CH4Be respectively as follows: with gas concentration
2.10×10-2、1.57×10-2、1.05×10-2、5.28×10-3、2.08×10-3、1.10×10-3、5.01×10-4、2.54
×10-4.Harmonic wave comparison result such as Fig. 2 that the fitting result of WMS-2f/1f waveform fitting method and measurement method of the present invention obtain
It is shown.As shown in Figure 2, work as CH4(> 1.10 × 10 when volumetric concentration is higher-3), WMS-2f/1f waveform is complete and regression criterion compared with
It is small.But with CH4Volumetric concentration reduces, and measurement signal-to-noise ratio reduction causes WMS-2f/1f waveform that obvious distortion occurs, at this time WMS-
2f/1f waveform fitting method is unsuitable to be used to calculate CH4Volumetric concentration.It is different from WMS-2f/1f waveform fitting method, this
Invention does not need complete shape information based on spectral absorption second harmonic feature extracting method, it is only necessary to extract second harmonic
Signal HαCentral peak height and the characteristic informations such as harmonic wave width.Actually due to HαThe noise of harmonic wave at centre-height
Than highest, and other peak heights are to HαThe influence of width is little, therefore method proposed by the present invention has higher signal-to-noise ratio.Even if
Work as CH4Volumetric concentration is 2.54 × 10-4When, HαHarmonic wave still has obvious peak height-peak width feature.Different CH4With gas concentration
Under, two methods measurement of concetration surveys result as shown in figs. 34.Work as CH4Volumetric concentration is greater than 2.08 × 10-3When, two methods are surveyed
Amount relative deviation is respectively less than 2%, (the < 1.10 × 10 when concentration further decreases-3), with measurement signal-to-noise ratio reduction, two kinds
The measured deviation of method all becomes larger, but the measurement result of harmonic characteristic extracting method of the present invention is closer to gas concentration.
As can be seen that using method proposed by the present invention without carrying out complicated least-squares iteration the Fitting Calculation and only needing
It is once filtered, reduces the requirement to hardware system；It still can be accurate when complete harmonic signal can not be obtained
Wave character point information is extracted, measurement lower limit is lower；Without using in database from ceofficient of spread, the collision of various other components
The excessive parameters such as ceofficient of spread, temperature-independent index, reduce the dependence to database parameter, and application range is wider.Measurement
As a result more accurate, compared with WMS-2f/1f waveform fitting method, the present invention proposes that the calculated result of method is more credible.
Claims (6)
1. a kind of gas concentration measuring method based on spectral absorption second harmonic feature extraction, which is characterized in that the measurement
Method the following steps are included:
Step 1, laser modulates optical maser wavelength, photodetector difference while scanning absorption line with high frequency sinusoidal signal
Measure the background light intensity signal I without absorption0With the transmitted light intensity signal I for having absorptiont；
Step 2, interference peak-to-peak signal of the measurement laser after solid etalon, and spacing is freely composed according to the solid etalon
Time domain light intensity signal is converted into frequency domain light intensity signal by FSR, thus obtain laser scanning relative wavenumbers variation relation V (t) and
The size a of modulation depth；
Step 3, transmitted light intensity signal I measurement obtainedtWith background light intensity signal I0Logarithm process is carried out, spectral absorption is obtained
Rate signal alpha (v)；
Step 4, locking phase filtering processing is carried out to spectral absorption signal alpha (v), obtains its corresponding second harmonic signal Hα；
Step 5, second harmonic signal H is calculated using peak-seeking algorithmαCenter peak heightsCombining laser scanning is with respect to wave
Second harmonic signal H is calculated in several variation relation V (t)αIn the secondary lobe width Λ of frequency domain；
Step 6, it establishes under any index of modulation, the corresponding second harmonic signal H of spectral absorption α (v)αCenter peak heights
And secondary lobe width Λ and integrated absorbance A and absorption line Lorentz broadening λLRelationship；
Step 7, absorption line Lorentz broadening λ is initializedL=λL0, it is absorbed the Gauss of spectral line according to known temperature computation
Broaden λG, in conjunction with the modulation depth a that measurement obtains, theoretical secondary lobe width Λ is calculated0；
Step 8, the theoretical secondary lobe width Λ that the secondary lobe width Λ and step 7 that judgment step 5 obtains are obtained0Whether following receipts are met
Hold back condition:
In formula, ε is preset convergence threshold；
If satisfied, the Lorentz broadening λ for the spectral line that is then absorbedL=λL0, the harmonic wave center peak heights that are obtained in conjunction with step 5
Integrated absorbance A is calculated, integrated absorbance A calculation formula is as follows:
If not satisfied, then updating λL0=λnew, return step 7；Wherein, λnewIt is the new value that used optimization algorithm generates, it is excellent
Changing algorithm can randomly select from algorithmic function each in MATLAB, and the function selected can generate a random new value；
Step 9, the concentration value of gas, the calculation formula of gas concentration value are calculated according to the integrated absorbance A being calculated
It is as follows:
In formula, P is gas stagnation pressure, and S (T) is that the spectral line line under temperature T is strong, and L is to absorb optical length.
2. the gas concentration measuring method according to claim 1 based on spectral absorption second harmonic feature extraction,
It is characterized in that: in step 3, the expression formula of logarithm process are as follows:
α (v)=- ln (It/I0)=A φ (v)
In formula: ItAnd I0Respectively laser transmitted light intensity and laser light incident light intensity；A is integrated absorbance；φ (v) is linear function.
3. the gas concentration measuring method according to claim 2 based on spectral absorption second harmonic feature extraction,
Be characterized in that: linear function φ (v) is the convolution of lorentzian curve function and Gaussian lineshape function, using Fu Yite linear function
Description, approximate expression are as follows:
φ (v)=cLφL(v)+cGφG(v)
In formula: φLAnd φGRespectively lorentzian curve function and Gaussian lineshape function；cLAnd cGRespectively Lorentz broadening λLWith
Gauss broadens λGWeight coefficient；v0For laser line centre frequency；λ is the full width at half maximum of absorption line；cL、cGWith the calculating of λ
Formula is as follows:
D=(λL-λG)/(λL+λG)
cL=0.6818817+0.6129331d-0.1838439d2-0.1156844d3
cG=0.3246017-0.6182531d+0.1768139d2+0.1210944d3
4. the gas concentration measuring method according to claim 1 based on spectral absorption second harmonic feature extraction,
It is characterized in that: in step 6, the corresponding second harmonic signal H of spectral absorption α (v)αCenter peak heightsWith integral extinction
Spend A and absorption line Lorentz broadening λLRelational expression are as follows:
In formula, I1For 1 rank modified Bessel function of the first kind, parameter m, cL、cGCalculation formula it is as follows:
D=(λL-λG)/(λL+λG)
cL=0.6818817+0.6129331d-0.1838439d2-0.1156844d3
cG=0.3246017-0.6182531d+0.1768139d2+0.1210944d3
M=2a/ λ.
5. the gas concentration measuring method according to claim 1 based on spectral absorption second harmonic feature extraction,
It is characterized in that: in step 6, the corresponding second harmonic signal H of spectral absorption α (v)αSecondary lobe width Λ and integrated absorbance A
With absorption line Lorentz broadening λLRelational expression are as follows:
In formula, r=cG/cL, p1=1.966194179, p2=0.390933340, p3=3.093996758, p4=
0.388999538, p5=0.806546476, p6=-0.289132222, p7=-1.775513203, p8=0.017671096,
p9=1.041510614, p10=-0.146221523, p11=-1.073806845.
6. the gas concentration measuring method according to claim 1 based on spectral absorption second harmonic feature extraction,
Be characterized in that: in step 9, the strong S of spectral line line (T) is varied with temperature and is indicated using following formula:
In formula, h is Planck constant, and h is the light velocity in vacuum, and k is Boltzmann constant, and E is low-level energy, T0=296K is
Reference temperature, v0For core frequency, the calculation method of partition function value when Q (T) is temperature T, E and Q (T) passes through inquiry
HITRAN2016 database obtains.
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