CN117451669A - High-sensitivity multi-gas real-time monitoring device and method - Google Patents
High-sensitivity multi-gas real-time monitoring device and method Download PDFInfo
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Abstract
The invention provides a high-sensitivity multi-gas real-time monitoring device and a high-sensitivity multi-gas real-time monitoring method, wherein the high-sensitivity multi-gas real-time monitoring device comprises a controller, the controller is connected with a main control circuit, the main control circuit is respectively connected with an indicating laser and a middle infrared laser array, the indicating laser and the middle infrared laser array are both connected with a multi-ventilation gas tank through optical fiber coupling structures, a photoelectric detector is arranged at the light outlet side of the multi-ventilation gas tank, and the photoelectric detector is connected with the main control circuit; the controller generates modulation signals with different frequencies to modulate the middle infrared laser array, and the indicating laser emitted by the indicating laser and the middle infrared laser emitted by the middle infrared laser array are coupled into a beam of coaxial light through an optical fiber coupling structure and are injected into the multi-pass gas tank; the photoelectric detector collects signals with target gas information through the multi-channel gas pool and simultaneously analyzes the concentration of various target gases through a signal conditioning algorithm. The invention has the characteristics of high sensitivity, low detection lower limit, real-time monitoring, convenient installation, strong anti-interference capability, no maintenance, wide application range and the like.
Description
Technical Field
The invention relates to the technical field of gas monitoring, in particular to a high-sensitivity multi-gas real-time monitoring device and method.
Background
Qualitative and quantitative detection of multi-component trace gas is increasingly paid attention to the application in various industries such as environmental pollution monitoring, industrial production and emission measurement, medical diagnosis, agriculture, food safety and the like, key gases in industrial processes are monitored in real time, and reliable operation of production and life safety of operators can be ensured. For example, C may be generated when the transformer fails 2 H 2 CO and CO 2 Waiting gas, and judging the safety condition and fault reason of the transformer according to the standard judgment standard; when coal is naturally oxidized, CO and C are generated 2 H 4 、C 2 H 2 And the like marking gas can judge the oxidation degree according to the gas type and the gas concentration, and early warning is sent out in advance to avoid spontaneous combustion of coal. Therefore, research on a high-sensitivity multi-component gas simultaneous monitoring technology has very important practical significance for safe production and life.
The traditional multi-component gas simultaneous monitoring technology is mostly gas chromatography, the sample injection characteristic determines that the gas detection time is long, continuous online monitoring is impossible, and the influence of temperature on the separation effect of a chromatographic column, the requirement of carrier gas consumption, periodic replacement of the chromatographic column and the like determine that the chromatographic technology is not suitable for long-term online monitoring application on site. Meanwhile, in multi-component measurement, due to the fact that the adopted detection principle and manufacturing process are limited and the chemical properties of the target gas to be measured are similar, the gas chromatograph cannot select the target gas to be ideal, and certain responses are generated to certain non-target gases in the use process, so that gas cross interference is caused. In addition, the traditional gas chromatography multi-gas detection device has huge volume and heavy equipment, for example, the external dimension of the oil chromatography analyzer special for an electric power system is 570 multiplied by 510 multiplied by 470 and mm, and the weight is more than 55kg, which is very unfavorable for the transportation of the instrument and the use of complex environments.
The mid-infrared laser gas detection technology is based on the beer lambert law, and has great application potential in the aspect of high-precision specificity detection of trace gas by utilizing the strong absorption characteristic and fingerprint identification characteristic of gas molecules in the mid-infrared band.
The invention patent with application number 202110395933.5 discloses a multi-component trace gas online detection device and method under a negative pressure state, wherein the measurement device comprises a laser control center, a plurality of tunable lasers, a polarization maintaining optical fiber, an optical switch, an optical beam splitter, an optical isolator, an optical fiber collimator, an air storage tank, a pressure control center, a plurality of high-reflection mirrors, a reaction tower, a vacuum pump, a plurality of ring-down cavities, a photoelectric detector, a central data processing and control center, a plurality of optical switches, a laser calibration center and a photoelectric converter. The invention provides a multi-component trace impurity gas online detection method with high sensitivity, which comprises the steps that gas to be detected is introduced into a ring-down cavity, a laser control center controls laser signals with different wavelengths sent by a tunable laser, an optical switch controls one wavelength of the laser signals to be divided into two laser signals with the energy ratio of 99:1 through an optical beam splitter, 99% energy of the laser signals are coupled into the ring-down cavity after passing through an optical isolator, 1% energy of the laser signals are transmitted into a calibration system, and according to the wavelength corresponding to the one wavelength, a photoelectric converter captures calibration information and transmits the calibration information to a central data processing and control center, a photoelectric detector captures the laser signals transmitted from the ring-down cavity and transmits the captured information to the central data processing and control center, so as to obtain ring-down time, and the concentration of the gas to be detected is calculated according to the ring-down time. However, the structure of the invention is complex, a plurality of precise structures such as a high-reflection mirror and a ring-down cavity are adopted, the mechanical stability is poor, and the invention is difficult to be suitable for a relatively complex field environment.
Disclosure of Invention
Aiming at the technical problems that the traditional gas chromatography multi-gas detection device is huge in size, low in sensitivity and incapable of performing real-time monitoring, the invention provides the high-sensitivity multi-gas real-time monitoring device and method.
In order to achieve the above purpose, the technical scheme of the invention is realized as follows: the high-sensitivity multi-gas real-time monitoring device comprises a controller, wherein the controller is connected with a main control circuit, the main control circuit is respectively connected with an indicating laser and a middle infrared laser array, the indicating laser and the middle infrared laser array are connected with a multi-ventilation gas tank through optical fiber coupling structures, a photoelectric detector is arranged at the light outlet side of the multi-ventilation gas tank, and the photoelectric detector is connected with the main control circuit; the middle infrared laser arrays are modulated by the modulating signals with different frequencies generated by the controller, so that the middle infrared lasers with different wavelengths work in parallel; the indicating laser emitted by the indicating laser and the middle infrared laser emitted by the middle infrared laser array are coupled into a beam of coaxial light through an optical fiber coupling structure and then are injected into the multi-pass gas tank; the controller collects signals with target gas information through the multi-channel gas pool according to the photoelectric detector, and simultaneously analyzes the concentration of various target gases through a signal conditioning algorithm.
Preferably, the controller is an MCU, and the MCU is connected with the main control circuit; the main control circuit is connected with a power supply, and the power supply provides proper working voltage for the controller, the indicating laser, the middle infrared laser array and the photoelectric detector through the main control circuit; the MCU receives the gas absorption signal filtered and amplified by the main control circuit and digitally demodulates the first harmonic signal 1fAnd second harmonic signal 2fAfter the harmonic signals are denoised by the adaptive wavelet threshold filtering algorithm, the harmonic signals are denoised by 2f/1fInverting the target gas concentration.
Preferably, the main control circuit comprises a power management module, a laser driving module, a laser temperature control module, a detector temperature control module and a signal processing module, wherein the power supply is used for providing proper working voltage for the controller and other modules through the power management module, the laser driving module and the laser temperature control module are both connected with the mid-infrared laser array, the laser driving module is used for providing stable driving current for the mid-infrared laser array and indicating the normal operation of the laser, and the laser temperature control module is used for controlling the working temperature of the mid-infrared laser; the detector temperature control module and the signal processing module are both connected with the photoelectric detector, and the detector temperature control module is used for controlling the photoelectric detector to work at a low temperature so as to ensure the high response rate of the photoelectric detector; the signal processing module is used for filtering and amplifying signals acquired by the photoelectric detector and transmitting the signals to the controller.
Preferably, the optical fiber coupling structure comprises a mid-infrared space optical fiber coupler and a mid-infrared optical fiber combiner, wherein the input end of the mid-infrared space optical fiber coupler is respectively connected with each mid-infrared laser of the mid-infrared laser array, the output end of the mid-infrared space optical fiber coupler is connected with the mid-infrared optical fiber combiner, and the mid-infrared optical fiber combiner is connected with the multi-channel gas tank; the infrared optical fiber beam combiner couples laser with different wavelengths emitted by the middle infrared laser array and the indicating laser emitted by the indicating laser into a coaxial light which is injected into the multi-pass gas tank.
Preferably, one end of the mid-infrared space optical fiber coupler is provided with an aspheric lens, the space light emitted by the mid-infrared laser is focused and coupled into an optical fiber by utilizing the focusing characteristics of short focal length and large numerical aperture of the aspheric lens, and the other end of the mid-infrared space optical fiber coupler is provided with an optical fiber interface adapter for connecting the mid-infrared optical fiber combiner; a focusing lens is arranged on the light outlet of the multi-way gas tank, and the center of the focusing lens and the light outlet are positioned on the same optical axis; the focusing lens is arranged on the incident window side of the photodetector.
Preferably, the multi-pass gas tank is a long-optical-path multi-pass gas tank, and one side of the long-optical-path multi-pass gas tank is provided with a light inlet and a light outlet; the laser after beam combination enters the long-optical-path multi-pass gas cell through the light inlet, and the light is output by the light outlet after being reflected for multiple times in the long-optical-path multi-pass gas cell and then focused by the focusing lens to reach the photoelectric detector; the long-optical-path multi-pass gas tank is a pumping type gas tank, and gas in a region to be detected is actively absorbed.
Preferably, the mid-infrared laser array comprises a plurality of quantum cascade lasers or interband cascade lasers which work in parallel, the wavelength range of visible light emitted by the indicating lasers is 390 nm-780nm, and the wavelength of the mid-infrared lasers is determined together according to the infrared spectrum of the target gas inquired by the HITRAN database and the absorption spectrum of the target gas measured by the Fourier spectrometer; the photoelectric detector is a tellurium-cadmium-mercury photoelectric detector, and the response range of the photoelectric detector is 2-12 mu m.
A monitoring method of a high-sensitivity multi-gas real-time monitoring device comprises the following steps:
step one: applying modulation signals with different frequencies to the middle infrared lasers which work in parallel, and coupling the middle infrared lasers with different modulation frequencies and the indication laser into a beam of coaxial laser which is injected into a long-path multi-pass gas cell through an optical fiber coupling structure;
step two: the coaxial laser is reflected for a plurality of times by the long-path multi-pass gas cell filled with the target gas, and then is emitted from the light outlet to reach the photoelectric detector through the focusing lens, the photoelectric detector converts the optical signal carrying the target gas information into an electric signal, and the electric signal enters the digital lock-in amplifier for demodulation after the pre-amplification, filtering and noise reduction treatment;
step three: the demodulated signals are simultaneously analyzed to obtain the concentration of various target gases through a signal conditioning algorithm.
Preferably, the modulating signals with different frequencies are sawtooth wave superposition sine waves, the modulating frequencies of the sine wave signals are different, and the modulating signals with different frequencies are applied to the corresponding mid-infrared lasers through different constant current source circuits; the modulating frequencies of sine wave signals of different mid-infrared lasers are not in integer multiple relation;
the method for demodulating the digital lock-in amplifier comprises the following steps: modulating signal frequency with a sine wave of a frequency and middle infrared laser Iw 1 The same synchronous reference signal and the amplified signal are multiplied by a multiplier to obtain a first harmonic direct current component of a modulated signal of the mid-infrared laser I and other mixed signals, and the signals with other frequencies are filtered by a low-pass filter to obtain a first harmonic signal 1 of the mid-infrared laser If 1 The method comprises the steps of carrying out a first treatment on the surface of the Then, the synchronous reference signal frequency is changed to 2w 1 Obtaining a second harmonic signal 2 of the mid-infrared laser If 1 The method comprises the steps of carrying out a first treatment on the surface of the Similarly, the first harmonic signal and the second harmonic signal corresponding to other lasers of the middle infrared laser array are obtained through digital demodulation.
Preferably, the signal conditioning algorithm further denoises harmonic signals output by the digital lock-in amplifier by adopting an adaptive wavelet threshold filtering algorithm;
the implementation method of the adaptive wavelet threshold filtering algorithm comprises the following steps: wavelet transformation is carried out on the noise-containing signal, the low-frequency coefficient and the high-frequency coefficient of each layer are extracted after wavelet decomposition, and the mean square error of noise is estimated by the high-frequency coefficient of the first layer; then, nonlinear threshold processing is carried out on the wavelet coefficient; and finally, performing wavelet inverse transformation to obtain pure first harmonic signals and second harmonic signals.
The nonlinear thresholding method comprises the following steps: the initial value of the threshold is determined by adopting a general threshold method proposed by Donoho: let the standard deviation of the noise signal be sigma, the signal length beNInitial threshold valueThe method comprises the steps of carrying out a first treatment on the surface of the Comparing the wavelet coefficient of the noise-containing signal with a selected threshold value, wherein the wavelet coefficient which is larger than or equal to the threshold value is kept unchanged, and the wavelet coefficient which is smaller than the threshold value is set to be zero; if the mean square error of the noise after noise elimination is larger than a preset value and the circulation times do not reach the maximum times limit, adjusting the threshold value and continuing the noise elimination process; otherwise, the denoising process is ended;
after noise elimination treatment by a self-adaptive wavelet threshold filtering algorithm, a first harmonic signal 1 with pure target gas is obtainedfAnd second harmonic signal 2fUsing first harmonic signals 1fFor second harmonic signal 2fNormalization processing is carried out, and finally according to 2f/1fThe linear corresponding relation of the ratio (C) to the concentration of the target gas is used for inverting the concentration of the target gas.
Compared with the prior art, the invention has the beneficial effects that: the combination of mid-infrared laser detection technology with tunable semiconductor laser absorption spectroscopy (Tunable Diode Laser Absorption Spectroscopy, TDLAS) can achieve highly sensitive detection of a variety of target gases. And:
(1) The installation is convenient: the whole device has compact structure, small volume, light weight and convenient movement and installation.
(2) The mechanical stability is good: the optical fiber coupling structure replaces the complex optical element beam combination optical path structure, improves the impact resistance of the device, and can be used for complex field environment.
(3) The selectivity is high, can accurately discern target gas: in a relatively wide mid-infrared spectrum range, the molecular spectral lines are less in overlapping and less in cross interference, and the high-precision quantitative analysis of the gas is realized by adopting mid-infrared laser to scan and analyze the fingerprint spectrum of the molecules.
(4) The sensitivity is high: the middle infrared laser gas detection technology and the TDLAS technology are combined, the function of extracting weak signals by utilizing the middle infrared band gas absorption line intensity and the harmonic detection technology is utilized, the detection lower limit can reach ppb level, and the high-sensitivity detection of gas is realized.
(5) And (3) real-time monitoring: the working mode that the middle infrared laser array runs in parallel and simultaneously calculates the concentration of a plurality of target gases is adopted, and the whole gas detection process only needs a few ms, so that real-time on-line monitoring is realized.
The invention has the characteristics of high sensitivity, low detection lower limit, real-time monitoring, convenient installation, strong anti-interference capability, no maintenance, wide application range and the like.
Drawings
In order to more clearly illustrate the embodiments of the invention or the technical solutions in the prior art, the drawings that are required in the embodiments or the description of the prior art will be briefly described, it being obvious that the drawings in the following description are only some embodiments of the invention, and that other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
Fig. 1 is a schematic structural view of the present invention.
Fig. 2 is a flow chart of denoising by the adaptive wavelet threshold filtering algorithm shown in fig. 1.
In the figure, 1 is a power supply, 2 is a controller, 3 is a main control circuit, 4 is an indicating laser, 5 is a middle infrared laser array, 6 is an optical fiber coupling structure, 7 is a multi-pass gas tank, 8 is a focusing lens, and 9 is a photoelectric detector.
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 any inventive effort, are intended to be within the scope of the invention.
Example 1
As shown in fig. 1, the invention provides a high-sensitivity multi-gas real-time on-line monitoring device, which comprises a power supply 1, wherein the power supply 1 is connected with a main control circuit 3, the main control circuit 3 is respectively connected with a controller 2, an indicating laser 4 and a middle infrared laser array 5, the indicating laser 4 and the middle infrared laser array 5 are both connected with a multi-pass gas tank 7 through an optical fiber coupling structure 6, a focusing lens 8 and a photoelectric detector 9 are arranged on the light outlet side of the multi-pass gas tank 7, and the photoelectric detector 9 is connected with the main control circuit 3.
The invention combines the mid-infrared laser gas detection technology and the TDLAS technology, utilizes sine waves with different frequencies to modulate the mid-infrared laser array 5, enables mid-infrared lasers with different wavelengths to work in parallel, and simultaneously analyzes the concentration of various target gases through a signal conditioning algorithm. The invention has the characteristics of high sensitivity, low detection lower limit, real-time monitoring, convenient installation, strong anti-interference capability, no maintenance, wide application range and the like.
The power supply 1 provides working voltage for the whole device, the power supply 1 is connected with the main control circuit 3, and the main control circuit 3 drives the controller 2, the main control circuit 3, the indicating laser 4, the middle infrared laser array 5, the photoelectric detector 9 and other modules to work normally.
The controller 2 is an MCU, is a control core of the whole device, and is electrically connected with the main control circuit 3. MCU has mainly 3 functionsThe energy can be: 1. the laser signal driving and temperature controlling function is used for controlling the working temperature of each laser of the middle infrared laser array 5 and the parameters of the modulating signals, specifically including the waveform, the frequency and the amplitude of the modulating signals, driving the middle infrared lasers to emit modulated laser through the main control circuit 3, monitoring the working state of the lasers, and closing the laser output in time when the working current is overlarge or the working temperature is overlarge; 2. a detector temperature control function for controlling the operating temperature of the photodetector 9; 3. the MCU receives the gas absorption signal filtered and amplified by the main control circuit 3, and digitally demodulates the first harmonic signal 1fAnd 2 nd harmonic signal 2fThen the harmonic signal is denoised by an adaptive wavelet threshold filtering algorithm, and finally 2f/1fInverting the target gas concentration.
The main control circuit 3 comprises a power management module, a laser driving module, a laser temperature control module, a detector temperature control module and a signal processing module, the power supply 1 provides proper working voltage for the controller 2 and other modules through the power management module, the laser driving module and the laser temperature control module are connected with the mid-infrared laser array 5, the controller 2 provides stable modulation current for the normal work of the mid-infrared laser through the laser driving module, the laser temperature control module adjusts the working temperature of the laser, and the laser driving module and the laser temperature control module act on the mid-infrared laser together to enable the mid-infrared laser to output mid-infrared modulation laser with specific wavelength. The detector temperature control module and the signal processing module are both connected with the photoelectric detector 9, and the detector temperature control module is used for controlling the photoelectric detector 9 to work at a low temperature so as to ensure the high response rate of the photoelectric detector. The signal processing module is used for filtering and amplifying the signal output by the photoelectric detector 9 and transmitting the signal to the controller 2.
The indicating laser 4 emits visible light with the wavelength range of 390 nm-780nm, the indicating laser emitted by the indicating laser 4 and the middle infrared laser emitted by the middle infrared laser array 5 are coupled into a beam of coaxial light through the optical fiber coupling structure 6 to emit, and the light path of the invisible light of the middle infrared is indicated by the light path of the visible light, so that the adjustment of the middle infrared is facilitatedThe outgoing angle of the infrared laser. The mid-infrared laser array 5 comprises a plurality of quantum cascade lasers or interband cascade lasers which work in parallel, the specific wavelength of the mid-infrared lasers is determined together according to the infrared spectrum of the target gas queried by the HITRAN database and the absorption spectrum of the target gas measured by the Fourier spectrometer, the principle is that the absorption line of the target gas is strong under the wavelength and no other gas interference exists, for example, when the high-sensitivity multi-gas real-time monitoring device is used for flue gas analysis of thermal power plants, garbage incineration plants, steel plants and the like, the mid-infrared laser array 5 consists of 3 mid-infrared lasers, and the center wavelengths are respectively 7.25 mu m, 5.26 mu m and 6.15 mu m and are respectively used for SO respectively 2 、NO、NO 2 High sensitivity and accuracy detection.
The optical fiber coupling structure 6 comprises a mid-infrared space optical fiber coupler and a mid-infrared optical fiber combiner, the input end of the mid-infrared space optical fiber coupler is respectively connected with each mid-infrared laser of the indicating mid-infrared laser array 5, the output end of the mid-infrared space optical fiber coupler is connected with the mid-infrared optical fiber combiner, and the mid-infrared optical fiber combiner is connected with the multi-channel gas tank 7. An aspheric lens is arranged at one end of the middle infrared space optical fiber coupler, space light emitted by the middle infrared laser is focused and coupled into an optical fiber by utilizing focusing characteristics of short focal length and large numerical aperture, an optical fiber interface adapter is arranged at the other end of the middle infrared space optical fiber coupler and is used for being connected with a middle infrared optical fiber beam combiner, and the middle infrared optical fiber beam combiner is used for coupling lasers with different wavelengths emitted by the middle infrared laser array 5 and indication lasers emitted by the indication lasers 4 into a coaxial light to be injected into the multi-pass gas tank 7. The optical fiber coupling structure of the optical fiber coupling structure 6 replaces a complex optical element beam combination optical path structure, so that the mechanical stability of the whole device is enhanced, the system integration is facilitated, and the volume of the device is reduced.
The multi-pass gas tank 7 is a long-optical-path multi-pass gas tank, and one side of the long-optical-path multi-pass gas tank is provided with a light inlet and a light outlet. The laser after beam combination enters the multi-pass gas tank through the light inlet, so that the optical path is lengthened in the gas tank with limited volume, the gas detection precision is improved, the light is output through the light outlet after multiple reflections in the gas tank, and the light reaches the photoelectric detector 9 through the focusing lens 8.
The long-optical-path multi-way gas tank 7 is a pumping type gas tank, actively absorbs the gas in the region to be detected, avoids the defect of low detection speed caused by a passive free diffusion detection mode, and can detect the occurrence and concentration change of the target gas more rapidly. The gas tank mainly comprises a light inlet, a light outlet, a gas inlet, a gas outlet, a cavity and the like, wherein the gas inlet and the gas outlet are positioned on the upper side of the cavity and are communicated with the cavity. The air inlet end of the air inlet is used for communicating the gas in the area to be detected, and a filter and an air pump are arranged at the air inlet. Dust, water vapor and the like in the working environment can be effectively removed by the filter, the influence on the detection result is avoided, the service life of the detection device is prolonged, and the maintenance cost is reduced. The gas outlet is used for timely discharging the detected gas, so that the influence of the detected gas which is not discharged on the subsequent detection result is avoided.
The light outlet side of the long-optical-path multi-way gas pool is provided with a focusing lens 8, and the center of the focusing lens 8 and the light outlet are on the same optical axis. The focusing lens 8 is arranged at the front end of the photoelectric detector 9, and focuses the laser output after being reflected for multiple times by the long-optical-path multi-pass gas cell to the photoelectric detector 9, so that the intensity of signals is enhanced, and the sensitivity of the system is improved.
The photoelectric detector 9 is a tellurium-cadmium-mercury photoelectric detector, the response range is 2-12 mu m, and the received optical signals are converted into electric signals, so that the function of simultaneously detecting the multicomponent target gas by a single detector is realized.
Example 2
As shown in FIG. 1, the invention provides a high-sensitivity multi-gas real-time on-line monitoring method, which comprises the following steps:
step one: and applying the modulation signals with different frequencies to the middle infrared lasers which work in parallel, and coupling the middle infrared lasers with different modulation frequencies into a beam of coaxial laser through an optical fiber coupling structure, wherein the beam of coaxial laser is injected into the long-path multi-pass gas cell.
Each of the mid-infrared lasers in the mid-infrared laser array 5 operates in parallel. First, the controller 2 generates modulated signals of different frequencies (sawtooth superimposed sine wave, sine waveThe wave signal modulation frequencies are different), then different modulation signals are applied to the corresponding mid-infrared lasers through the laser driving modules, namely, the modulation signals 1 are applied to the mid-infrared lasers 51 through the constant current source circuits 1 in the laser driving modules, and the sine wave signal modulation frequency isw 1 The method comprises the steps of carrying out a first treatment on the surface of the The modulating signal 2 is applied to the mid-infrared laser 52 by the constant current source circuit 2 in the laser driving module, and the modulating frequency of the sine wave signal isw 2 The method comprises the steps of carrying out a first treatment on the surface of the … …; the modulating signal n is applied to the mid-infrared laser 5n through a constant current source circuit n in the laser driving module, and the modulating frequency of the sine wave signal isw n And the sine wave signal modulation frequency of the laserw 1 、w 2 、……、w n The two are not in integer multiple relation. Taking the example of the high-sensitivity multi-gas real-time monitoring device when being used for coal spontaneous combustion early warning, the middle infrared laser array 5 consists of 3 middle infrared lasers, namely lasers 51, 52 and 53, the central wavelengths of which are respectively 4.6 mu m, 10.5 mu m and 3.03 mu m, and the high-sensitivity multi-gas real-time monitoring device is respectively used for detecting CO and C 2 H 4 And C 2 H 2 The modulation frequencies of sine wave signals of the 3 lasers are respectivelyw 1 、w 2 、w 3 Whereinw 2 3.5 timesw 1 ，w 3 3.5 timesw 2 Other relationships are possible as long as they are not integer multiples of each other. Laser beams with different modulation frequencies are coupled into a beam of coaxial laser through an optical fiber coupling structure 6, and the beam of coaxial laser enters a long-path multi-pass gas cell 7.
Step two: the coaxial laser is reflected for multiple times in the long-path multi-pass gas cell 7 filled with target gas and then is emitted from a light outlet, then enters the focusing lens 8 and the photoelectric detector 9, the photoelectric detector 9 converts optical signals carrying target gas information into electric signals, and the electric signals enter the digital lock-in amplifier for demodulation after being subjected to pre-amplification, filtering, noise reduction and other treatments.
Step three: and simultaneously analyzing the concentration of a plurality of target gases through a signal conditioning algorithm.
The demodulation algorithm specifically comprises the following steps: when the receiving end processes the signal, the amplified signal is input into the digital receiverIn the word lock-in amplifier, a frequency and a mid-infrared laser 51 are used to modulate the signal frequencyw 1 The same synchronous reference signal and the amplified signal are multiplied by a multiplier to obtain a first harmonic DC component of the modulated signal of the mid-infrared laser 51 and other mixed signals, and the signals with other frequencies are filtered by a low-pass filter to obtain a first harmonic signal 1 of the laser 51f 1 . Then, the synchronous reference signal frequency is changed to 2w 1 Obtaining a second harmonic signal 2 of the modulated signal of the mid-infrared laser 51f 1 . Similarly, C can be obtained by changing the corresponding frequency of the reference signal 2 H 4 And C 2 H 2 A first harmonic signal and a second harmonic signal of the same.
In the detected signal output by the digital phase-locked amplifier, due to the influence of the modulation frequencies of all lasers of the middle infrared laser array 5 and the influence of residual amplitude modulation, noise still exists to a certain extent, and the harmonic signal output by the phase-locked amplifier is further denoised by adopting an adaptive wavelet threshold filtering algorithm.
As shown in fig. 2, the implementation method of the adaptive wavelet threshold filtering algorithm is as follows: firstly, carrying out wavelet transformation on a noise-containing signal, extracting low-frequency coefficients and high-frequency coefficients of each layer after wavelet decomposition, and then estimating the mean square error of noise by the high-frequency coefficients of the first layer; the wavelet coefficients are then non-linearly thresholded. And finally, performing wavelet inverse transformation to obtain pure first harmonic signals and second harmonic signals. The nonlinear thresholding method comprises the following steps: the general threshold method proposed by Donoho is adopted to determine the initial value of the threshold, and the standard deviation of the noise signal is set asσThe signal length isNInitial threshold sizeThe wavelet coefficients of the noisy signal are compared with a selected threshold, the wavelet coefficients greater than or equal to the threshold remain unchanged, and the wavelet coefficients less than the threshold are set to zero. If the mean square error of the noise after noise elimination is larger than a preset value and the circulation times do not reach the maximum times limit, adjusting the threshold value and continuing the noise elimination process; reverse-rotationIf so, the noise cancellation process ends.
After noise elimination treatment by a self-adaptive wavelet threshold filtering algorithm, a first harmonic signal 1 with pure target gas is obtainedfAnd second harmonic signal 2fThen using the first harmonic signal 1fFor second harmonic signal 2fNormalization processing is carried out, and finally, the MCU2 is used for processing the data according to the data of 2f/1fThe linear corresponding relation of the ratio (C) to the concentration of the target gas is used for inverting the concentration of the target gas.
The foregoing description of the preferred embodiments of the invention is not intended to be limiting, but rather is intended to cover all modifications, equivalents, alternatives, and improvements that fall within the spirit and scope of the invention.
Claims (10)
1. The high-sensitivity multi-gas real-time monitoring device is characterized by comprising a controller (2), wherein the controller (2) is connected with a main control circuit (3), the main control circuit (3) is respectively connected with an indication laser (4) and a middle infrared laser array (5), the indication laser (4) and the middle infrared laser array (5) are connected with a multi-channel gas tank (7) through an optical fiber coupling structure (6), a photoelectric detector (9) is arranged at the light outlet side of the multi-channel gas tank (7), and the photoelectric detector (9) is connected with the main control circuit (3); the middle infrared laser array (5) is modulated by the modulating signals with different frequencies generated by the controller (2), so that the middle infrared lasers with different wavelengths work in parallel; the indicating laser emitted by the indicating laser (4) and the middle infrared laser emitted by the middle infrared laser array (5) are coupled into a beam of coaxial light through the optical fiber coupling structure (6) and then are injected into the multi-pass gas tank (7); the controller (2) collects signals with target gas information through the multi-channel gas pool according to the photoelectric detector (9), and simultaneously analyzes the concentration of various target gases through a signal conditioning algorithm.
2. The high-sensitivity multi-gas real-time monitoring device according to claim 1, wherein the power supply (1) is connected with the main control circuit (3) and is supplied to the main control circuit (3)The controller (2), the indicating laser (4), the middle infrared laser array (5) and the photoelectric detector (9) provide proper working voltages; the controller (2) is an MCU, receives the gas absorption signal filtered and amplified by the main control circuit (3), and digitally demodulates the first harmonic signal 1fAnd second harmonic signal 2fAfter the harmonic signals are denoised by the adaptive wavelet threshold filtering algorithm, the harmonic signals are denoised by 2f/1fInverting the target gas concentration.
3. The high-sensitivity multi-gas real-time monitoring device according to claim 1 or 2, wherein the main control circuit (3) comprises a power management module, a laser driving module, a laser temperature control module, a detector temperature control module and a signal processing module, the power management module provides proper working voltages for the controller (2) and other modules, the laser driving module and the laser temperature control module are both connected with the mid-infrared laser array (5), the laser driving module provides stable driving current for the normal operation of the mid-infrared laser array (5) and the indication laser (4), and the laser temperature control module controls the working temperature of the mid-infrared laser; the detector temperature control module and the signal processing module are both connected with the photoelectric detector (9), and the detector temperature control module is used for controlling the photoelectric detector (9) to work at a low temperature so as to ensure the high response rate of the photoelectric detector (9); the signal processing module is used for filtering and amplifying signals acquired by the photoelectric detector (9) and transmitting the signals to the controller (2) for digital demodulation.
4. The high-sensitivity multi-gas real-time monitoring device according to claim 3, wherein the optical fiber coupling structure (6) comprises a mid-infrared space optical fiber coupler and a mid-infrared optical fiber combiner, the input end of the mid-infrared space optical fiber coupler is respectively connected with each mid-infrared laser of the mid-infrared laser array (5), the output end of the mid-infrared space optical fiber coupler is connected with the mid-infrared optical fiber combiner, and the mid-infrared optical fiber combiner is connected with the multi-pass gas cell (7); the infrared optical fiber beam combiner couples laser light with different wavelengths emitted by the middle infrared laser array (5) and indication laser light emitted by the indication laser (4) into a coaxial light which is injected into the multi-pass gas tank (7).
5. The high-sensitivity multi-gas real-time monitoring device according to claim 4, wherein one end of the mid-infrared space optical fiber coupler is provided with an aspheric lens, the space light emitted by the mid-infrared laser is focused and coupled into an optical fiber by utilizing the focusing characteristics of short focal length and large numerical aperture of the aspheric lens, and the other end of the mid-infrared space optical fiber coupler is provided with an optical fiber interface adapter for connecting a mid-infrared optical fiber combiner; a focusing lens (8) is arranged at the light outlet side of the multi-way gas pool (7), and the center of the focusing lens (8) and the light outlet are on the same optical axis; the focusing lens (8) is arranged on the incident window side of the photodetector (9).
6. The high-sensitivity multi-gas real-time monitoring device according to any one of claims 1, 2, 4 and 5, wherein the multi-gas cell (7) is a long-optical-path multi-gas cell, and a light inlet and a light outlet are arranged on one side of the long-optical-path multi-gas cell; the laser after beam combination enters a long-optical-path multi-way gas tank through a light inlet, and light is output by a light outlet after being reflected for multiple times in the long-optical-path multi-way gas tank and then focused by a focusing lens (8) to reach a photoelectric detector (9); the long-optical-path multi-pass gas tank is a pumping type gas tank, and gas in a region to be detected is actively absorbed.
7. The high-sensitivity multi-gas real-time monitoring device according to claim 1, wherein the mid-infrared laser array (5) comprises a plurality of quantum cascade lasers or interband cascade lasers which are arranged in parallel, the wavelength range of visible light emitted by the indicating laser (4) is 390 nm-780nm, and the wavelength of the mid-infrared laser is jointly determined according to the infrared spectrum of the target gas queried by the HITRAN database and the absorption spectrum of the target gas measured by the Fourier spectrometer; the photoelectric detector (9) is a tellurium-cadmium-mercury photoelectric detector, and the response range of the photoelectric detector (9) is 2-12 mu m.
8. The monitoring method of the high-sensitivity multi-gas real-time monitoring device according to any one of claims 1, 2, 4, 5 or 7, characterized by comprising the steps of:
step one: applying modulation signals with different frequencies to each middle infrared laser which works in parallel, and coupling the middle infrared lasers with different modulation frequencies and the indication laser into a beam of coaxial laser which is injected into a long-path multi-pass gas cell through an optical fiber coupling structure;
step two: the coaxial laser is reflected for many times by the long-path multi-pass gas pool (7) filled with target gas, and then is emitted from the light outlet to reach the photoelectric detector (9) through the focusing lens (8), the photoelectric detector (9) converts optical signals carrying target gas information into electric signals, and the electric signals enter the digital lock-in amplifier for demodulation after pre-amplification, filtering and noise reduction treatment;
step three: the demodulated signals are simultaneously analyzed to obtain the concentration of various target gases through a signal conditioning algorithm.
9. The monitoring method according to claim 8, wherein the modulated signals with different frequencies are sawtooth wave superimposed sine waves, the modulated frequencies of the sine wave signals are different, and the modulated signals with different frequencies are applied to the corresponding mid-infrared lasers through different constant current source circuits; the modulating frequencies of sine wave signals of different mid-infrared lasers are not in integer multiple relation;
the method for demodulating the digital lock-in amplifier comprises the following steps: modulating signal frequency with a sine wave of a frequency and mid-infrared laser (51)w 1 The same synchronous reference signal and the amplified signal are multiplied by a multiplier to obtain a first harmonic direct current component of a modulated signal of the mid-infrared laser (51) and other mixed signals, and the signals with other frequencies are filtered by a low-pass filter to obtain a first harmonic signal 1 of the mid-infrared laser (51)f 1 The method comprises the steps of carrying out a first treatment on the surface of the Then, the synchronous reference signal frequency is changed to 2w 1 Obtaining the mid-infrared laserSecond harmonic signal 2 of the device (51)f 1 The method comprises the steps of carrying out a first treatment on the surface of the Similarly, the first harmonic signal and the second harmonic signal corresponding to other lasers of the middle infrared laser array are obtained through digital demodulation.
10. The method according to claim 9, wherein the signal conditioning algorithm further denoises harmonic signals output from the digital lock-in amplifier using an adaptive wavelet threshold filtering algorithm;
the implementation method of the adaptive wavelet threshold filtering algorithm comprises the following steps: wavelet transformation is carried out on the noise-containing signal, the low-frequency coefficient and the high-frequency coefficient of each layer are extracted after wavelet decomposition, and the mean square error of noise is estimated by the high-frequency coefficient of the first layer; then, nonlinear threshold processing is carried out on the wavelet coefficient; finally, carrying out wavelet inverse transformation to obtain pure first harmonic signals and second harmonic signals;
the nonlinear thresholding method comprises the following steps: the initial value of the threshold is determined by adopting a general threshold method proposed by Donoho: let the standard deviation of the noise signal be sigma, the signal length beNInitial threshold valueThe method comprises the steps of carrying out a first treatment on the surface of the Comparing the wavelet coefficient of the noise-containing signal with a selected threshold value, wherein the wavelet coefficient which is larger than or equal to the threshold value is kept unchanged, and the wavelet coefficient which is smaller than the threshold value is set to be zero; if the mean square error of the noise after noise elimination is larger than a preset value and the circulation times do not reach the maximum times limit, adjusting the threshold value and continuing the noise elimination process; otherwise, the denoising process is ended;
after noise elimination treatment by a self-adaptive wavelet threshold filtering algorithm, a first harmonic signal 1 with pure target gas is obtainedfAnd second harmonic signal 2fUsing first harmonic signals 1fFor second harmonic signal 2fNormalization processing is carried out, and finally according to 2f/1fThe linear corresponding relation of the ratio (C) to the concentration of the target gas is used for inverting the concentration of the target gas.
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