CN120064206B - A trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy - Google Patents
A trace hydrogen concentration sensing system based on TDLAS absorption spectroscopyInfo
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Abstract
The invention relates to the technical field of hydrogen concentration detection, and particularly discloses a trace hydrogen concentration sensing system based on a TDLAS absorption spectrum, which is used for solving the problems of low detection sensitivity, narrow linear range, large system drift and dependence on external calibration existing in the TDLAS hydrogen detection, and comprises a laser emission module and a photoelectric detection module, wherein the laser emission module and the photoelectric detection module are connected through a double-light-path absorption module, the photoelectric detection module is connected with a harmonic extraction module, the harmonic extraction module is connected with a normalization processing module, and the normalization processing module is connected with a concentration inversion module; according to the invention, by constructing a symmetrical double-light-path structure and adopting a harmonic signal normalization and differential compensation algorithm and combining a high-pressure reference gas pool and a piecewise fitting model, the high linearity, high sensitivity and calibration-free concentration detection of hydrogen in the range of 0.01% -100% are realized, and the hydrogen trace identification capability is improved.
Description
Technical Field
The invention relates to the technical field of hydrogen concentration detection, in particular to a trace hydrogen concentration sensing system based on TDLAS absorption spectrum.
Background
Hydrogen is used as a new energy carrier with clean, renewable and high energy density, and is widely applied to the fields of energy storage, transportation, industrial manufacture, fuel cells and the like, however, because the hydrogen has the characteristics of no color, no smell, extremely easy diffusion, inflammability and explosiveness, and is difficult to perceive when in leakage, and great threat is brought to environmental safety, therefore, a hydrogen sensing system with high sensitivity, wide dynamic detection range and quick response capability is developed, which is a key support for the development of the hydrogen energy by utilizing industrial safety, the currently applied hydrogen sensing technology mainly comprises electrochemical, thermal conductivity, capacitance, resistance, optoacoustic and other methods, the Chinese patent publication No. CN1187948914 mainly discloses a trace hydrogen concentration detection device, which is used for carrying out catalytic oxidation on trace hydrogen into water vapor by designing a double-channel optoacoustic cell, detecting trace steam by using optoacoustic spectrum technology, further obtaining corresponding hydrogen concentration, and carrying out processing by differential operation of voltage signals, ensuring that an excellent linear relation is presented between an output electric signal and the hydrogen concentration to be detected, realizing ppb level trace hydrogen concentration detection, and although the device has certain advantages in the aspects of low cost and system integration, the device realizes that the trace hydrogen concentration detection is in the aspects of ppm detection and the high-level, the linear response is more than 100%, the laser thermal absorption coefficient is more than the characteristics of the laser light absorption, the laser light absorption device has the characteristics of high-sensitivity, the laser light absorption dynamic response is more than the characteristics of the laser light absorption, and the laser absorption device is more stable, and the laser absorption device is more stable, and the thermal absorption device is wide in the thermal absorption, and stable, and has the characteristics of the thermal absorption, and high-infrared absorption, and high-absorption performance, and high-sensitivity, and high sensitivity. How to improve the detection sensitivity, compress the systematic error, and realize the accurate quantification without external calibration becomes the technical bottleneck to be solved in the TDLAS detection of the current hydrogen.
Disclosure of Invention
The technical problem to be solved by the invention is to provide a trace hydrogen concentration sensing system based on TDLAS absorption spectrum, which realizes high linearity, high sensitivity and calibration-free concentration detection of hydrogen within 0.01% -100% by constructing a symmetrical double-light-path structure and adopting a harmonic signal normalization and differential compensation algorithm and combining a high-pressure reference gas pool and a piecewise fitting model, and improves the hydrogen trace identification capability.
In order to achieve the above purpose, the present invention provides the following technical solutions:
A trace hydrogen concentration sensing system based on TDLAS absorption spectrum comprises a laser emission module and a photoelectric detection module, wherein the laser emission module and the photoelectric detection module are connected through a double-light-path absorption module, the photoelectric detection module is connected with a harmonic extraction module, the harmonic extraction module is connected with a normalization processing module, the normalization processing module is connected with a concentration inversion module, the laser emission module comprises a distributed feedback coordinative diode laser, the emission wavelength of the laser emission module covers a hydrogen near infrared 2121.8nm main absorption line, the double-light-path absorption module comprises a first gas pool and a second gas pool, the first gas pool is filled with target to-be-detected gas, the second gas pool is filled with hydrogen with known concentration and is in a high-pressure state with the pressure being greater than or equal to 8 standard atmospheres, the laser emission beam forms a target light path and a reference light path through the first gas pool and the second gas pool respectively, the photoelectric detection module comprises two photoelectric detectors, the harmonic extraction module respectively receives projected laser signals of the target light path and the reference light path, the harmonic extraction module adopts a phase-locked amplifier to extract a first fundamental wave signal of the target light path projected laser signal, a first second harmonic signal, extracts a second fundamental wave signal of the reference light path projected signal, a second harmonic signal of the second harmonic signal, the normalization processing module is used for carrying out normalization processing, the first harmonic signal and a compensation ratio value of the first harmonic signal to be combined with a target absorption module to a trace absorption coefficient, and a dynamic compensation ratio value is formed by the target absorption module, and a dynamic ratio compensation ratio value is based on the target absorption module.
As a further scheme of the invention, one side of the laser is provided with a beam splitter, the beam splitter divides an emission beam of the laser into a first beam and a second beam, the other side of the beam splitter is provided with a first optical component and a second optical component, and the photoelectric detection module comprises a first photoelectric detector and a second photoelectric detector.
As a further scheme of the invention, the first optical component comprises a first off-axis parabolic mirror, a first optical filter is arranged above the first off-axis parabolic mirror, the first optical filter is positioned below the first photoelectric detector, a first window piece is arranged on one side, far away from the laser, of the first off-axis parabolic mirror, a second window piece is arranged on the other side of the first window piece, the first gas pool is positioned between the first window piece and the second window piece, and a first full-angle reflecting mirror is arranged on the other side of the second window piece.
As a further aspect of the present invention, the first beam path of the laser split by the beam splitter is:
The first light beam firstly passes through the middle opening of the first off-axis parabolic mirror and the first window sheet, enters the first gas pool to complete the first absorption, then the laser beam passes through the second window sheet and then irradiates the first full-angle reflecting mirror, after being reflected by the original path, enters the first gas pool again through the second window sheet to conduct the second absorption, the reflected light beam passes through the first window sheet and then enters the parabolic surface of the first off-axis parabolic mirror, and the reflected light is focused on the photosensitive surface of the first photoelectric detector after passing through the first optical filter.
As a further scheme of the invention, the second optical component comprises a second off-axis parabolic mirror, a second optical filter is arranged above the second off-axis parabolic mirror, the second optical filter is positioned below the second photoelectric detector, a third window piece is arranged on one side, far away from the laser, of the second off-axis parabolic mirror, a fourth window piece is arranged on the other side of the third window piece, a second gas pool is positioned between the third window piece and the fourth window piece, and a second full-angle reflecting mirror is arranged on the other side of the fourth window piece.
As a further aspect of the present invention, the second beam path of the laser split by the beam splitter is:
The second light beam firstly passes through the middle opening of the second off-axis parabolic mirror and the third window sheet, enters the second gas pool to complete the first absorption, then the laser beam passes through the fourth window sheet and then irradiates the second full-angle reflecting mirror, after being reflected by the original path, enters the second gas pool again through the fourth window sheet to conduct the second absorption, the reflected light beam passes through the third window sheet and then enters the parabolic surface of the second off-axis parabolic mirror, and the reflected light is focused on the photosensitive surface of the second photoelectric detector after passing through the second optical filter.
As a further scheme of the invention, the normalization processing module adopts a fundamental wave normalization algorithm to process the first second harmonic signal extracted from the target light path, obtains a main signal ratio, and introduces a compensation ratio of a reference signal, and the specific process comprises the following steps: the first second harmonic signal is divided by the corresponding first fundamental signal to form a primary signal ratio, and the second harmonic signal is divided by the corresponding second fundamental signal to form a compensation ratio for the reference signal.
As a further scheme of the invention, in a concentration inversion module, a preset fitting model is established on the basis of a tunable diode laser absorption spectrometry harmonic detection theory, a piecewise linear regression function is adopted, and an independent fitting sub-model is established for 0.01% -1%, 1% -10% and 10% -100% of a set concentration interval, wherein the trace hydrogen concentration fitting function of each section is as follows:
;
Wherein: For the index of the concentration interval, Is the firstThe trace hydrogen concentration of each concentration interval is fitted to the function value,、Are all fitting coefficients, are obtained through experiments,As a result of the first second harmonic signal,Is the first fundamental wave signal;
meanwhile, the compensation ratio extracted from the reference light path is introduced, harmonic response errors caused by laser fluctuation and mirror pollution are corrected, and the trace hydrogen concentration fitting function formula of each corrected concentration section is as follows:
;
Wherein: To be corrected after The trace hydrogen concentration of each concentration interval is fitted to the function value,、The second harmonic signal and the second fundamental wave signal are respectively obtained.
As a further scheme of the invention, the effective absorption optical path of the second gas pool is more than or equal to 2 meters, and the filled gas is high-purity gas with the volume fraction of more than 99.99%.
As a further scheme of the invention, the first photoelectric detector and the second photoelectric detector are high-sensitivity detectors with response bandwidths higher than 20kHz and linear dynamic ranges higher than or equal to 90dB, and photosensitive surfaces of the high-sensitivity detectors are respectively perpendicular to reflection axes of a target light path and a reference light path and are used for receiving laser signals which are coupled and focused to the first optical filter and the second optical filter after twice absorption and reflection.
The invention discloses a trace hydrogen concentration sensing system based on TDLAS absorption spectrum, which has the technical effects that:
According to the trace hydrogen concentration sensing system based on the TDLAS absorption spectrum, provided by the invention, a double-light-path structure is constructed to respectively guide laser beams to pass through a first gas pool filled with gas to be detected and a second gas pool filled with high-pressure high-purity hydrogen, so that symmetrical absorption path configuration of a target light path and a reference light path is realized, a beam splitter, an off-axis parabolic mirror, a light filter and a full-angle reflecting mirror are combined to construct an enhanced absorption light path with multiple transmission and reflection, the amplitude and the signal-to-noise ratio of harmonic signals are improved, fundamental wave signals and second harmonic signals in the target light path and the reference light path are synchronously extracted, a main signal ratio and a reference compensation ratio are formed through normalization processing, the measurement errors caused by laser power fluctuation, light path disturbance and device response drift are dynamically eliminated, a hydrogen concentration result covering a range of 0.01% -100% is output based on the ratio pair, the rapid measurement capability under the complex working condition is not required, and the quantitative detection precision, the system stability and the industrial adaptability of the trace hydrogen are improved.
Drawings
FIG. 1 is a system diagram of the present invention;
FIG. 2 is a schematic view of the optical path of the present invention;
FIG. 3 shows the present invention And (3) with、AndA relation diagram between the green linesAnd (3) withRelationship curve, blue lineAndA relationship curve;
The device comprises a laser emitting module, a photoelectric detection module, a double-light-path absorption module, a 4-harmonic extraction module, a 5-normalization processing module, a 6-concentration inversion module, a 8-laser, a 9-phase lock amplifier, a 10-beam splitter, a 11-first optical component, a 12-second optical component, a 21-first photoelectric detector, a 22-second photoelectric detector, a 31-first gas pool, a 32-second gas pool, a 111-first off-axis parabolic mirror, a 112-first optical filter, a 113-first window, a 114-second window, a 115-first full-angle parabolic mirror, a 121-second off-axis parabolic mirror, a 122-second optical filter, a 123-third window, a 124-fourth window, and a 125-second full-angle parabolic mirror.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
Example 1
As shown in fig. 1, the trace hydrogen concentration sensing system based on TDLAS absorption spectrum provided by the invention comprises a laser emitting module 1 and a photoelectric detection module 2, wherein the laser emitting module 1 and the photoelectric detection module 2 are connected through a double-light-path absorption module 3, the photoelectric detection module 2 is connected with a harmonic extraction module 4, the harmonic extraction module 4 is connected with a normalization processing module 5, the normalization processing module 5 is connected with a concentration inversion module 6, the laser emitting module 1 comprises a distributed feedback type coordinative diode laser 8, the emission wavelength of the laser emission beam covers a near infrared 2121.8nm main absorption line of hydrogen, the double-light-path absorption module 3 comprises a first gas tank 31 and a second gas tank 32, the first gas tank 31 is filled with target gas to be detected, the second gas tank 32 is filled with hydrogen with known concentration and is in a high-pressure state with the pressure being more than or equal to 8 standard atmospheres, a laser emission beam respectively forms a target light path and a reference light path through the first gas tank 31 and the second gas tank 32, the photoelectric detection module 2 comprises two photoelectric detectors which respectively receive projection laser signals of the target light path and the reference light path, the harmonic extraction module 4 adopts a phase-locked amplifier 9 to extract a first fundamental wave signal and a first second harmonic signal of the projection laser signals of the target light path, extracts a second fundamental wave signal and a second harmonic signal of the projection laser signals of the reference light path, the normalization processing module 5 carries out normalization processing on the first second harmonic signal to form a main signal ratio, and introducing a compensation ratio of a reference light path, dynamically compensating the power drift of the light source and the disturbance of the light path, and outputting the trace concentration value of the hydrogen in the target gas by a concentration inversion module 6 based on the main signal ratio and the compensation ratio and combining a preset fitting model and absorption coefficient parameters.
According to the invention, through the trace hydrogen concentration sensing system based on the TDLAS absorption spectrum as shown in fig. 1, a double-light-path absorption module with symmetrical structure is introduced between a laser emission module and a photoelectric detection module, a target light path and a high-voltage reference light path are respectively constructed through a first gas tank 31 and a second gas tank 32, so that the laser can obtain equivalent light path absorption and multiple reflection enhancement signals in the two paths, a photoelectric detector with high response bandwidth is matched to collect two-path transmission signals, a phase-locked amplifier is used for extracting a first fundamental wave signal and a first second harmonic signal of the target light path, and a second fundamental wave signal and a second harmonic signal of the reference light path, and a main signal ratio and a reference compensation ratio are respectively formed through a normalization processing module, so that the laser power instability, light path fluctuation and system response deviation can be dynamically counteracted, and absorption information such as gas concentration can be extracted from the main signal ratio.
When detecting the first fundamental information and the first second harmonic signal using TDLAS techniques, the transmission of the laser beam through the absorbing gas is described using Lambert-Beer law:
;
wherein, the 、The intensity of the light incident and received respectively,Is a factor of the reception efficiency and,、、The absorption coefficient, optical path length and gas concentration at the absorption center wavelength, respectively;
The injection current of the laser 8 is sine wave modulated so that the output power of the laser 8 is modulated in response to the wavelength of the incident laser light, and when the modulation center of the laser light wavelength is aligned with the gas absorption center, the laser light wavelength and intensity are expressed by equations one and two:
Equation one: ;
Equation two: ;
Wherein: in order to absorb the central wavelength of the light, Is the width of the wavelength modulation,Is the modulation frequency of the signal to be modulated,As a function of the time variable,Is thatThe laser wavelength at the moment in time,Is thatThe intensity of the light at the moment in time,For the intensity modulation index due to the laser current modulation,The system can be developed into a Fourier series comprising a first harmonic amplitude and a second harmonic amplitude, the Fourier series is substituted into the optical depth of hydrogen, and the direct current component of the laser power received in the first equation is given by the following formula:
Equation three: ;
Equation four: ;
Equation five: ;
wherein, the 、、Respectively the received direct current component of the laser power, the first harmonic current and the second harmonic current,Is a constant value, and is used for the treatment of the skin,For the optical depth of the hydrogen gas,In direct proportion to the optical depth of hydrogen throughRatio cancellation of (2)AndTaking into account the interaction between the width of the wavelength modulation and the intensity modulation index in equations one and two due to the characteristics of the laser diode, determining the absorption characteristics of hydrogen is critical to obtain a hydrogen concentration close to the true value, the absorption coefficient of the lorentz line typeBy fourier expansion, as in equation six:
Wherein In order to absorb the full width at half maximum of the line,As provided by Wahlquist, the number of the channels,,,And in the expression of (2)Is directly applicable to second harmonic detection, in order to measure trace concentrations (ppm level), maximization when configuring optimal system parametersThe value and the modulation width are adjusted,The sensitivity of the sensor reaches the detection limit, and the free calibration method is used for eliminating the fluctuation of illumination intensity caused by other factors except the gas absorptionAndTo measure the gas concentration in ppm,And (3) with、AndThe relationship between them is plotted in FIG. 3, the adjustmentTo maximize the value of (a)AndWhen (when)AndAt the time of the value of 0,AndThe dimensionless confidence of (2) reaches a maximum value when the scale factorAnd,Is equal to the dimensionless confidence of,Is equal to the dimensionless confidence of。
The emitting part uses a distributed feedback laser diode emitting at 2121.83nm, the frequency is modulated in the form of a sine wave of 10kHz, and the modulation center is locked at the center of the hydrogen maximum absorption line (2121.83 nm). In order to lock the laser diode on the maximum absorption line of hydrogen, the injection current and temperature are fixed by monitoring the maximum absorption on the line. The collimated laser beam is split by the beam splitter 10 and passed into a first gas cell 31 and a second gas cell 32.
The concentration inversion module presets a fitting function group to realize linear inversion output of the hydrogen concentration within the range of 0.01% -100%, the minimum detection limit of the system under the integration time of 30 seconds is better than 0.0055% (55 ppm), the linear fitting goodness in the whole range can reach 0.9995, the system has the advantages of no need of external calibration, high sensitivity, high stability and online real-time detection capability, and is suitable for industrial scenes such as hydrogen fuel quality control, storage, transportation and leakage early warning and the like.
The beam splitter 10 is disposed on one side of the laser 8, the beam splitter 10 splits the light beam emitted by the laser 8 into a first light beam and a second light beam, the first optical component 11 and the second optical component 12 are disposed on the other side of the beam splitter 10, and the photo-detection module 2 includes a first photo-detector 21 and a second photo-detector 22.
It should be specifically noted that, as shown in fig. 2, the first optical assembly 11 includes a first off-axis parabolic mirror 111, a first optical filter 112 is disposed above the first off-axis parabolic mirror 111, the first optical filter 112 is located below the first photodetector 21, a first window 113 is disposed on a side of the first off-axis parabolic mirror 111 away from the laser 8, a second window 114 is disposed on another side of the first window 113, the first gas pool 31 is located between the first window 113 and the second window 114, and a first full-angle reflector 115 is disposed on another side of the second window 114. The first beam path of the laser 8 split by the beam splitter 10 is:
The first light beam firstly passes through the middle opening of the first off-axis parabolic mirror 111 and the first window 113, enters the first gas tank 31 to complete the first absorption, then the laser beam passes through the second window 114 and then is emitted to the first full-angle reflecting mirror 115, after being reflected by the original path, the first light beam enters the first gas tank 31 again through the second window 114 to carry out the second absorption, the reflected light beam passes through the first window 113 and then enters the parabolic surface of the first off-axis parabolic mirror 111, and the reflected light is focused on the photosensitive surface of the first photoelectric detector 21 after passing through the first optical filter 112.
According to the invention, after the direction of a first light beam output by a laser 8 through a beam splitter 10 is changed by a plane mirror, the first light beam sequentially passes through a middle opening of a first off-axis parabolic mirror 111 and a first window 113 to enter a first gas pool 31 to finish first gas absorption, then passes through a second window 114 to irradiate to a first full-angle reflecting mirror 115 and reflect along an original path to form a second gas absorption path, finally, the second gas absorption path is reflected by the first off-axis parabolic mirror 111 and focused to a first photoelectric detector 21 through a first optical filter 112, so that two-way enhanced absorption and high-efficiency signal acquisition are realized, the collimation and focusing characteristics of the first off-axis parabolic mirror 111 are utilized, the problem of central shielding is effectively avoided while the laser coupling efficiency and the signal transmission stability are ensured, the amplitude of a low-concentration hydrogen absorption signal is improved, the response sensitivity of a second harmonic is improved, meanwhile, the optical filter is used for isolating background stray light and only allowing the transmission of a target wave band, the signal to be transmitted, the system can obtain the minimum detection light beam of less than 0.0055% under the 30 seconds integration time, and the system has the best detection space-limited space, the high-concentration and the high-quality design and the stability is required, and the system is high in the overall-quality and the quality is designed.
It should be noted that, as shown in fig. 2, the second optical assembly 12 includes a second off-axis parabolic mirror 121, a second optical filter 122 is disposed above the second off-axis parabolic mirror 121, the second optical filter 122 is located below the second photodetector 22, a third window 123 is disposed on a side of the second off-axis parabolic mirror 121 away from the laser 8, a fourth window 124 is disposed on another side of the third window 123, the second gas cell 32 is located between the third window 123 and the fourth window 124, and a second full-angle reflector 125 is disposed on another side of the fourth window 124. The second beam path of the laser 8 split by the beam splitter 10 is:
the second light beam is redirected by the plane mirror, firstly passes through the middle opening of the second off-axis parabolic mirror 121 and the third window 123, enters the second gas pool 32 to complete the first absorption, then the laser beam passes through the fourth window 124 and then is emitted to the second full-angle reflecting mirror 125, after being reflected by the original path, enters the second gas pool 32 again through the fourth window 124 to carry out the second absorption, the reflected light beam passes through the third window 123 and then enters the parabolic surface of the second off-axis parabolic mirror 121, and the reflected light is focused on the photosensitive surface of the second photoelectric detector 22 after passing through the second optical filter 122.
The second optical component 12 in the invention is used as a reference light path structure, and forms symmetrical layout with the first optical component 11, and adopts a second off-axis parabolic mirror 121 to realize laser collimation and reflection focusing, a second light beam output by the beam splitter 10 enters a second gas pool 32 through a third window 123 after entering through a middle opening, first penetrates through known high-concentration hydrogen, then is projected to a second full-angle reflecting mirror 125 through a fourth window 124 to form 180 DEG reflection and returns along an original path, passes through the second gas pool 32 again, and returns to the second off-axis parabolic mirror 121 and is focused through a second optical filter 122 to a second photoelectric detector 22 after forming a double-path absorption path; the reference light path takes high-pressure standard hydrogen (more than or equal to 8 atm) as a filling medium, stably outputs an absorption response signal, effectively improves the amplitude and resolution capability of compensating harmonic signals through double-pass reflection enhancement, is structurally symmetrical to a target light path, effectively ensures the consistency of main signals and reference signals on geometric paths, mirror group reflection and optical path conditions, ensures that a good alignment relation exists between harmonic normalization ratio and compensation ratio, is beneficial to eliminating systematic errors caused by fluctuation of a laser 8, optical path drift and nonlinearity of a device, and further enhances the robustness of differential fitting in a concentration inversion module 6, and improves the long-term stability and calibration independence of trace hydrogen concentration measurement in a complex environment.
Example 2
Embodiment 2 of the present invention differs from embodiment 1 in that this embodiment describes a normalization processing module 5, a concentration inversion module, a first gas cell 31, a first photodetector 21, and a second photodetector 22 of a trace hydrogen concentration sensing system based on TDLAS absorption spectrum.
It should be noted that, the normalization processing module 5 processes the first second harmonic signal extracted from the target optical path by adopting a fundamental wave normalization algorithm to obtain a main signal ratio, and introduces a compensation ratio of the reference signal.
The normalization processing module 5 processes the first second harmonic signal extracted from the target light path through a fundamental wave normalization algorithm, takes the corresponding first fundamental wave signal as a normalization factor to form a main signal ratio, further introduces the ratio of the second harmonic signal to the second fundamental wave signal If 2 in the reference light path as a compensation ratio, and counteracts the system error by constructing a double-ratio structure, wherein the processing mode physically realizes the dynamic elimination of uncertain factors such as laser initial power fluctuation, lens pollution, inconsistent modulation depth, reflector reflectivity change and the like, so that the main signal ratio is only related to the gas concentration, thereby improving the stability and the repeatability of the system under the condition without external calibration; the algorithm not only improves the normalization precision of signal processing, but also realizes the real-time drift self-compensation capability by introducing a dual-path differential mechanism, effectively reduces the measurement drift and response lag in the long-time operation process, ensures that the linear fitting goodness R2 of the whole system is more than or equal to 0.999 in the concentration range of 0.01% -100%, has the minimum detection limit of better than 55ppm under the integration time of 30 seconds, and meets the comprehensive requirements of industrial-grade trace hydrogen detection on high sensitivity, anti-interference performance and long-term maintenance-free performance.
In the concentration inversion module 6, a preset fitting model is established on the basis of a tunable diode laser absorption spectroscopy harmonic detection theory, a piecewise linear regression function is adopted, and an independent fitting sub-model is established for the set concentration interval sections of 0.01% -1%, 1% -10% and 10% -100%, and trace hydrogen concentration fitting function forms of the sections are as follows:
;
Wherein: For the index of the concentration interval, Is the firstThe trace hydrogen concentration of each concentration interval is fitted to the function value,、Are all fitting coefficients, are obtained through experiments,As a result of the first second harmonic signal,Is the first fundamental wave signal;
Meanwhile, the compensation ratio extracted from the reference light path is introduced to correct harmonic response errors caused by fluctuation and mirror pollution of the laser 8, and the trace hydrogen concentration fitting function formula of each corrected concentration section is as follows:
;
Wherein: To be corrected after The trace hydrogen concentration of each concentration interval is fitted to the function value,、The second harmonic signal and the second fundamental wave signal are respectively obtained.
The method comprises the steps of dividing a hydrogen concentration detection range into three concentration intervals of 0.01% -1%, 1% -10% and 10% -100% by adopting a piecewise linear regression model established on the basis of a TDLAS harmonic detection theory in a concentration inversion module 6, respectively constructing an independent fitting function in each interval to process a nonlinear relation between a main signal ratio and concentration in different concentration intervals, so that the model keeps good local fitting precision and intersegmental continuity in the whole detection range, simultaneously introducing a compensation ratio in a reference light path as a correction term, and realizing dynamic offset of systematic errors such as laser power fluctuation, inconsistent modulation depth, reflector pollution, optical filter attenuation and the like in a mathematical manner by constructing a differential form, so that concentration output is determined only by absorption spectrum response, an external calibration curve is not needed, high-precision maintenance-free hydrogen concentration measurement can be realized, and experimental results show that the method can keep fitting goodness of the whole concentration range of 0.01% -100%, the minimum detection limit of the system under the integration time of 30 seconds is better than 0.5%, the sensitivity of hydrogen detection is remarkably improved, the sensitivity and the linear safety and the stability requirements of hydrogen detection are high, and the method is suitable for industrial safety and stability.
The effective absorption optical path of the second gas cell 32 is 2 m or more, and the gas to be filled is high-purity gas with a volume fraction of 99.99% or more.
The invention provides an effective absorption optical path larger than or equal to 2 meters for a second gas pool 32, which aims to improve the total absorption capacity of gas molecules to laser by prolonging the interaction path of laser and target gas, thereby enhancing the amplitude and signal-to-noise ratio of a second harmonic signal, and the invention aims to ensure that the ratio response of the second gas pool 32 is provided with high singleness and spectral line consistency when the concentration is inverted, and avoid the interference of absorption peaks of impurity gases such as CH 4、CO2 or H 2 O and the like in wave bands under the detection condition of trace (ppm level), and ensure that the fitting model is corresponding to the absorption coefficient, if the optical path is insufficient, the harmonic response amplitude can be submerged by system noise, so that the detection limit rises, and the equivalent optical path larger than or equal to 2 meters (can be realized through multi-path reflection), on the premise of keeping the compact structure of the system, and in addition, the target gas filled in the second gas pool 32 is high-purity hydrogen with volume fraction larger than or equal to 99.99 percent.
It should be noted that, the first photodetector 21 and the second photodetector 22 are high-sensitivity detectors with response bandwidths higher than 20kHz and linear dynamic ranges greater than or equal to 90dB, and the photosensitive surfaces thereof are perpendicular to the reflection axes of the target optical path and the reference optical path, respectively, and are used for receiving the laser signals coupled and focused to the first optical filter 112 and the second optical filter 122 after twice absorption and reflection.
The first photoelectric detector 21 and the second photoelectric detector 22 adopted in the invention are high-sensitivity devices, have signal response bandwidths higher than 20kHz and linear dynamic ranges higher than or equal to 90dB, can respond to wavelength modulation laser signals output by a laser at a modulation frequency of 10-20 kHz in real time, ensure that harmonic components are not filtered or distorted in a signal acquisition link, can completely reserve first harmonic components and second harmonic components contained in the laser signals in the modulation process by the high-bandwidth characteristics, provide stable input sources for accurately extracting fundamental wave and second harmonic signals by a subsequent phase-locked amplifier, simultaneously ensure that laser transmission signals with different intensity levels can still be accurately responded under the condition that the concentration span is extremely large (0.01% -100%), avoid signal saturation or signal distortion when the concentration is low, ensure that the laser signals focused by the two detectors are perpendicular to reflection axes of a target light path and a reference light path respectively, ensure that the laser signals subjected to twice absorption, reflection and off-axis parabolic mirror focusing can be perpendicular to the detection planes, improve the photoelectric conversion efficiency and reduce the angle deviation, ensure that the response ratio of the system is higher than the maximum and the highest, and the reliability of the system is higher than the reliability of the signal detection performance is lower than the maximum and the reliability is higher than the reliability of the signal detection system, and the signal is lower than the signal detection performance is lower than the signal quality.
In summary, as described in connection with embodiments 1 and 2, the system provided by the invention respectively guides the laser beam to pass through the first gas tank 31 filled with the gas to be detected and the second gas tank 32 filled with the high-pressure high-purity hydrogen by constructing a dual-optical path structure, realizes symmetrical absorption path configuration of the target optical path and the reference optical path, constructs an enhanced absorption optical path of multiple transmission and reflection by combining the beam splitter, the off-axis parabolic mirror, the optical filter and the full-angle reflecting mirror, improves the amplitude and the signal-to-noise ratio of harmonic signals, synchronously extracts fundamental wave signals and second harmonic signals in the target optical path and the reference optical path, forms a main signal ratio and a reference compensation ratio by normalization processing, dynamically eliminates measurement errors caused by laser power fluctuation, optical path disturbance and device response drift, outputs a hydrogen concentration result covering a range of 0.01% -100% based on the ratio pair, combines a preset multi-section fitting model and an absorption coefficient parameter, has high linearity, low detection limit, does not need external calibration, adapts to rapid measurement capability under complex working conditions, and improves quantitative detection accuracy, system stability and industrial adaptability of trace hydrogen.
In addition, the distributed feedback tunable diode laser can be replaced by a narrow linewidth laser covering other near infrared or infrared wave bands according to different target absorption line spectrums so as to adapt to detection requirements of other trace gases, the optical path length and the air pressure parameters of the first air tank 31 and the second air tank 32 can be enhanced by selecting a multi-channel reflecting mirror group or an annular multi-path air chamber, higher equivalent absorption length or air compression ratio can be realized, the distributed feedback tunable diode laser adapts to application scenes with lower detection limits, the first off-axis parabolic mirror 111 and the second off-axis parabolic mirror 121 in the first optical component 11 and the second optical component 12 can be replaced by an ellipsoidal reflecting mirror or a spherical lens and reflecting mirror combination so as to adapt to different optical path layout and equipment volume requirements, meanwhile, the laser collimation and focusing functions are kept, the harmonic extraction module can be replaced by adopting a high-order harmonic extraction, third harmonic analysis or a digital phase-locked amplification technology according to the precision requirement and the signal frequency characteristic of a detection system, the normalization processing and concentration inversion module can be realized by an FPGA, a DSP or an embedded AI chip, and the intelligent concentration calibration and real-time communication function can be realized by connecting the FPGA, the intelligent concentration calibration and the intelligent concentration calibration module. Therefore, various alternatives, equivalents, or combinations of techniques falling within the scope of the claims should be considered as protecting the invention.
Claims (8)
1. The trace hydrogen concentration sensing system based on the TDLAS absorption spectrum comprises a laser emitting module (1) and a photoelectric detection module (2), and is characterized in that the laser emitting module (1) and the photoelectric detection module (2) are connected through a double-light-path absorption module (3), the photoelectric detection module (2) is connected with a harmonic extraction module (4), the harmonic extraction module (4) is connected with a normalization processing module (5), the normalization processing module (5) is connected with a concentration inversion module (6), the laser emitting module (1) comprises a laser (8), the double-light-path absorption module (3) comprises a first gas tank (31) and a second gas tank (32) which are respectively filled with target gas to be detected and hydrogen with known concentration and at the atmospheric pressure of 8 or more standard atmosphere, the laser emission beam passes through and respectively forms a symmetrical target and a reference light path, the photoelectric detection module (2) comprises two photoelectric detectors which respectively receive laser signals of the target light path and the reference light path, the harmonic extraction module (4) respectively extracts a first fundamental wave signal and a first second harmonic wave signal of the target light path and a second fundamental wave signal and a second harmonic wave signal of the reference light path, the normalization processing module (5) carries out fundamental wave normalization on the first second harmonic wave signal to form a main signal ratio, and simultaneously introduces a compensation ratio to dynamically compensate the power drift and the light path disturbance of the light source, the concentration inversion module (6) outputs a hydrogen trace concentration value based on the main signal ratio and the compensation ratio and in combination with a fitting model, a beam splitter (10) is arranged on one side of the laser (8), the beam splitter (10) divides an emission beam of the laser (8) into a first beam and a second beam, a first optical component (11) and a second optical component (12) are arranged on the other side of the beam splitter (10), and the photoelectric detection module (2) comprises a first photoelectric detector (21) and a second photoelectric detector (22);
In a concentration inversion module (6), a preset fitting model is established on the basis of a tunable diode laser absorption spectrometry harmonic detection theory, an independent fitting sub-model is established for 0.01% -1%, 1% -10% and 10% -100% of a set concentration interval by adopting a piecewise linear regression function, and the trace hydrogen concentration fitting function of each section is formed by:
;
Wherein: For the index of the concentration interval, Is the firstThe trace hydrogen concentration of each concentration interval is fitted to the function value,、Are all fitting coefficients, are obtained through experiments,As a result of the first second harmonic signal,Is the first fundamental wave signal;
meanwhile, the compensation ratio extracted from the reference light path is introduced, harmonic response errors caused by fluctuation of the laser (8) and mirror pollution are corrected, and the trace hydrogen concentration fitting function formula of each corrected concentration section is as follows:
;
Wherein: To be corrected after The trace hydrogen concentration of each concentration interval is fitted to the function value,、The second harmonic signal and the second fundamental wave signal are respectively obtained.
2. The TDLAS absorption spectrum based trace hydrogen concentration sensing system of claim 1, wherein the first optical assembly (11) comprises a first off-axis parabolic mirror (111), a first optical filter (112) is arranged above the first off-axis parabolic mirror (111), the first optical filter (112) is located below the first photodetector (21), a first window (113) is arranged on one side of the first off-axis parabolic mirror (111) away from the laser (8), a second window (114) is arranged on the other side of the first window (113), the first gas cell (31) is located between the first window (113) and the second window (114), and a first full angle reflector (115) is arranged on the other side of the second window (114).
3. A trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy according to claim 2, wherein the first beam path of the laser (8) split by the beam splitter (10) is:
The first light beam firstly passes through the middle opening of the first off-axis parabolic mirror (111) and the first window (113) and enters the first gas tank (31) to finish the first absorption, then the laser beam passes through the second window (114) and then is emitted to the first full-angle reflecting mirror (115), after being reflected by the original path, the first light beam enters the first gas tank (31) again through the second window (114) to perform the second absorption, the reflected light beam passes through the first window (113) and then enters the paraboloid of the first off-axis parabolic mirror (111), and the reflected light is focused on the photosensitive surface of the first photoelectric detector (21) after passing through the first optical filter (112).
4. A trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy according to claim 1, wherein the second optical assembly (12) comprises a second off-axis parabolic mirror (121), a second optical filter (122) is arranged above the second off-axis parabolic mirror (121), the second optical filter (122) is located below the second photodetector (22), a third window (123) is arranged on a side of the second off-axis parabolic mirror (121) away from the laser (8), a fourth window (124) is arranged on the other side of the third window (123), the second gas cell (32) is located between the third window (123) and the fourth window (124), and a second full angle reflector (125) is arranged on the other side of the fourth window (124).
5. A trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy according to claim 4, wherein the second beam path of the laser (8) split by the beam splitter (10) is:
The second light beam firstly passes through the middle opening of the second off-axis parabolic mirror (121) and the third window (123) and enters the second gas pool (32) to finish the first absorption, then the laser beam passes through the fourth window (124) and then is emitted to the second full-angle reflecting mirror (125), after being reflected by the original path, the reflected light beam enters the second gas pool (32) again through the fourth window (124) to perform the second absorption, the reflected light beam passes through the third window (123) and then enters the parabolic surface of the second off-axis parabolic mirror (121), and the reflected light is focused on the photosensitive surface of the second photoelectric detector (22) after passing through the second optical filter (122).
6. The trace hydrogen concentration sensing system based on the TDLAS absorption spectrum according to claim 1, wherein the normalization processing module (5) processes a first second harmonic signal extracted from a target optical path by adopting a fundamental wave normalization algorithm to obtain a main signal ratio, and introduces a compensation ratio of a reference signal.
7. A trace hydrogen concentration sensing system based on TDLAS absorption spectroscopy according to claim 1, wherein the effective absorption path of the second gas cell (32) is greater than or equal to 2 meters, and the gas filled is a high purity gas with a volume fraction greater than 99.99%.
8. The TDLAS absorption spectrum-based trace hydrogen concentration sensing system according to claim 1, wherein the first photodetector (21) and the second photodetector (22) are high-sensitivity detectors with response bandwidths higher than 20kHz and linear dynamic ranges higher than or equal to 90dB, and photosensitive surfaces thereof are perpendicular to reflection axes of the target optical path and the reference optical path, respectively, and are used for receiving laser signals coupled and focused to the first optical filter (112) and the second optical filter (122) after two absorption and reflection.
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