CN110987870A - System and method for real-time monitoring of gas concentration based on wavelength modulation spectroscopy - Google Patents
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Abstract
一种监测气体浓度的系统，包括分布式反馈激光器、信号发生模块、角锥棱镜、离轴抛物面镜、光电探测模块、信号传输模块、锁相放大模块和信号处理模块，其中：所述分布式反馈激光器输出的激光经准直后通过离轴抛物面镜中的中心孔和目标气体的待测区域，由角锥棱镜反射回至离轴抛物面镜，返回的激光汇聚到所述光电探测模块的光敏区；信号传输模块将所述光电探测模块采集到的电信号传输给锁相放大模块；锁相放大模块利用所述信号发生模块提供的参考信号将所述光电探测模块采集到的电信号进行解调；所述信号处理模块通过滤波器提取2f信号，并利用滤波器对所述提取的信号进行平滑滤波，从而为后续的算法处理提供2f信号和直流信号。
A system for monitoring gas concentration, comprising a distributed feedback laser, a signal generation module, a corner prism, an off-axis parabolic mirror, a photoelectric detection module, a signal transmission module, a phase-locked amplification module and a signal processing module, wherein: the distributed The laser output from the feedback laser is collimated and passes through the central hole in the off-axis parabolic mirror and the area to be measured of the target gas, and is reflected back to the off-axis parabolic mirror by the corner cube prism. The signal transmission module transmits the electrical signal collected by the photoelectric detection module to the lock-in amplifier module; the lock-in amplifier module uses the reference signal provided by the signal generation module to decode the electrical signal collected by the photoelectric detection module. The signal processing module extracts the 2f signal through a filter, and uses the filter to smooth the extracted signal, so as to provide the 2f signal and the DC signal for subsequent algorithm processing.
Description
Technical Field
The invention relates to the field of exhaust emission, in particular to a system and a method for monitoring gas concentration in real time based on a wavelength modulation spectrum technology.
Background
The tunable laser absorption spectrum technology can realize in-situ on-line measurement of parameters such as component concentration, temperature, pressure, speed and the like by utilizing the laser absorption characteristics of components in the gas to be measured. Compared with other technologies, the laser absorption spectrum technology has the advantages of accurate measurement, high response speed, non-invasive induction and the like, and has wide application prospect in the field of atmospheric environment detection. The tunable laser absorption spectroscopy technology is mainly divided into a direct absorption spectroscopy technology and a wavelength modulation spectroscopy technology.
The working principle of the direct absorption spectrum technology is that a certain scanning current is injected into a laser, the laser wave band emitted by the laser is driven to completely cover the wave band of a gas absorption peak, the laser passes through an absorption path containing gas to be detected and is absorbed by the gas at a specific position, an absorbed optical signal is converted into an electric signal through a photoelectric detector, and the characteristics of the absorption peak are provided by means of baseline fitting and the like. The direct absorption spectrum technology system is simple and easy to realize, but the measuring result is easily influenced by the emergent light intensity of the laser and the like, and the stability of the measuring result is poor. Therefore, the stability and the measurement accuracy of the system are often improved by the wavelength modulation spectrum technology.
The working principle of the wavelength modulation spectrum is that a high-frequency sinusoidal signal with the frequency f is modulated on the scanning current of a laser, laser emitted by the laser is absorbed by gas to be measured and then converted into an electric signal by a photoelectric detector, a phase-locked amplifier carries out phase-sensitive detection processing by utilizing the mutual irrelevance between an input signal and a noise signal, the electric signal is demodulated by a same-frequency (1f) or frequency-doubling (nf) signal and a complete useful signal is extracted from noise, and the concentration of the gas to be measured can be obtained by the relation between the amplitude of a harmonic signal and the concentration of the gas to be measured. The value of the frequency f is dozens of kHz to hundreds of kHz, and the frequency and the amplitude of the high-frequency sinusoidal modulation signal are changed, so that the optical noise in the direct absorption spectrum technology can be effectively reduced. Meanwhile, the wavelength modulation spectrum technology moves the information to be detected to a high-frequency wave band, so that 1/f low-frequency noise in the system is effectively inhibited, and higher detection precision is obtained.
In actual measurement, the intensity of laser is easily influenced by temperature, dust, vibration and the like to cause intensity change, the inversion concentration error is large only according to a 2f signal, a reference light path is generally arranged by adding a reference cell and the like to eliminate the influence of light intensity change on a measurement result, and the system complexity is high.
Disclosure of Invention
In view of the above, the main object of the present invention is to provide a system and a method for monitoring gas concentration, which are intended to at least partially solve at least one of the above technical problems.
In order to achieve the above object, as an aspect of the present invention, a system for monitoring gas concentration is provided, including a distributed feedback laser, a signal generating module, a corner cube prism, an off-axis parabolic mirror, a photodetection module, a signal transmission module, a phase-locked amplification module, and a signal processing module, wherein:
laser output by the distributed feedback laser is collimated, then passes through a central hole in the off-axis parabolic mirror and a region to be detected of target gas, is reflected back to the off-axis parabolic mirror by the pyramid prism, and the returned laser is converged to a photosensitive region of the photoelectric detection module;
the signal transmission module transmits the electric signal acquired by the photoelectric detection module to the phase-locked amplification module;
the phase-locked amplification module demodulates the electric signal acquired by the photoelectric detection module by using the reference signal provided by the signal generation module;
the signal processing module extracts the 2f signal through a filter and utilizes the filter to carry out smooth filtering on the extracted signal, thereby providing the 2f signal and the direct current signal for subsequent algorithm processing.
The signal generation module generates a sawtooth signal and a sine signal, and modulates the sine signal onto the sawtooth signal as a driving signal of the distributed feedback laser;
the photoelectric detection module is used for converting the received optical signal into an electric signal containing gas concentration information to obtain a direct absorption signal.
The system also comprises a temperature control module, wherein the temperature control module is used for controlling the working temperature of the distributed feedback laser, the internal secondary temperature control can accurately control the temperature to be the required temperature, the error is +/-0.001 ℃, and the stability of the output wavelength of the distributed feedback laser is effectively ensured;
the system also comprises a current driving module, wherein the current driving module converts the modulated voltage signal into a current signal and performs current tuning on the distributed feedback laser, so that the wavelength range of laser emitted by the distributed feedback laser is ensured to completely cover the absorption wavelength of the target gas.
The system utilizes the direct current signal to carry out normalization processing on the 2f signal, a reference air chamber and a reference light path do not need to be arranged inside the system, and the complexity of the system is low.
Wherein the signal processing module extracts an absorption line of the gas by using an FIR filter.
The signal processing module is used for carrying out normalization processing on the 2f signal by using the direct current signal, and quickly inverting the concentration by establishing a second-order relation between the peak value and the concentration of the normalized 2f signal.
The inversion method comprises the following specific steps:
pre-collecting 2f signals and direct absorption signals of different concentrations in a gas area to be detected, and carrying out the following processing:
extracting the maximum value of the 2f wave crest at the position known as the absorption peak, extracting the left wing minimum value and the right wing minimum value of the 2f wave trough at the positions of two wings of the absorption peak, subtracting the average value of the left wing minimum value and the right wing minimum value from the maximum value to obtain the peak value of the 2f absorption peak, and marking as 2 fpeak;
because the number of points of the direct absorption signal is known to be equal to that of the 2f signal, selecting a certain section of direct absorption signal at the position without an absorption peak, and averaging the section of direct absorption signal to be marked as dcmean;
normalizing the peak value 2fpeak of the 2f absorption peak by using the mean value dcmean of the direct absorption signal to obtain a normalized 2f value which is marked as 2 fmean;
establishing a nonlinear relation between the concentration and 2 fmean;
and flushing the gas with the concentration to be measured into the region of the gas to be measured, calculating to obtain the normalized 2f value, and substituting the value into the established nonlinear relation to obtain the true value of the concentration of the gas to be measured.
The signal generation module, the signal transmission module, the phase-locking amplification module and the signal processing module are integrated on the same circuit board.
The temperature control module and the current driving module are integrated on the same circuit board.
As another aspect of the present invention, there is also provided a method of monitoring a gas concentration, comprising the steps of:
the system is electrified and automatically started, the signal generation module generates waveform voltage, and the current driving module converts a voltage signal into current to drive the laser to generate a laser signal;
the laser signal passes through a target area of the gas to be detected through a central hole of the off-axis parabolic mirror, and the pyramid prism reflects the laser signal to the off-axis parabolic mirror;
the off-axis parabolic mirror converges laser on the photoelectric detection module, the photoelectric detection module converts an optical signal into a voltage signal, the signal transmission module collects the voltage signal and transmits the voltage signal to the phase-locked amplification module, and the phase-locked amplification module analyzes a 2f signal;
judging whether the light intensity meets the requirements, if not, alarming and judging whether the light intensity meets the requirements; and if the requirements are met, acquiring a 2f signal, and obtaining the gas concentration by adopting a quick inversion step.
The rapid inversion method comprises the following steps:
pre-collecting 2f signals and direct absorption signals of different concentrations in a gas area to be detected, and carrying out the following processing:
extracting the maximum value of the 2f wave crest at the position known as the absorption peak, extracting the left wing minimum value and the right wing minimum value of the 2f wave trough at the positions of two wings of the absorption peak, subtracting the average value of the left wing minimum value and the right wing minimum value from the maximum value to obtain the peak value of the 2f absorption peak, and marking as 2 fpeak;
because the number of points of the direct absorption signal is known to be equal to that of the 2f signal, selecting a certain section of direct absorption signal at the position without an absorption peak, and averaging the section of direct absorption signal to be marked as dcmean;
normalizing the peak value 2fpeak of the 2f absorption peak by using the mean value dcmean of the direct absorption signal to obtain a normalized 2f value which is marked as 2 fmean;
establishing a nonlinear relation between the concentration and 2 fmean;
and flushing the gas with the concentration to be measured into the region of the gas to be measured, calculating to obtain the normalized 2f value, and substituting the value into the established nonlinear relation to obtain the true value of the concentration of the gas to be measured.
Based on the technical scheme, compared with the prior art, the system and the method for monitoring the gas concentration have at least one of the following beneficial effects:
1. according to the method, the absorption signal of the environmental gas is collected in real time to serve as a background curve to be deducted, a reference gas chamber is not required to be arranged in the system, and the influence of the airtightness of the gas chamber, the adsorbability of the gas and the like on the measurement result is avoided;
2. the system of the invention utilizes the FIR filter to extract the absorption spectral line of the gas by selecting proper filter coefficients, thereby reducing the noise interference;
3. according to the method, the 2f signal is normalized by using the direct current signal, the influence of laser light intensity jitter, dust and the like on the light intensity is eliminated, and the concentration is quickly inverted by establishing a second-order linear relation between the peak value and the concentration of the normalized 2f signal;
4. in the system, the functions of the signal generation module, the phase-locking amplification module, the signal processing module and the like are integrated on one circuit board, and the functions of the current driving module and the temperature control module of the laser are integrated on one circuit board, so that the modularization and the integration of the system are facilitated.
Drawings
FIG. 1 is a flowchart of the operation of a hardware system according to an embodiment of the present invention;
FIG. 2 is a flow chart of a monitoring method according to an embodiment of the present invention;
FIG. 3 is a spectrum of the absorption peak and concentration of CO in example 1 of the present invention;
FIG. 4 is a second order nonlinear fit of CO concentration to 2fmean in example 1 of the present invention;
FIG. 5 shows CO in example 2 of the present invention2A spectrum of absorption peaks versus concentration;
FIG. 6 shows CO in example 2 of the present invention2Second order nonlinear fit plot of concentration to 2 fmean.
Detailed Description
In order that the objects, technical solutions and advantages of the present invention will become more apparent, the present invention will be further described in detail with reference to the accompanying drawings in conjunction with the following specific embodiments.
The invention discloses a system and a method for monitoring gas concentration in real time based on a wavelength modulation spectrum technology, which centralize the functions of a signal generation module, a phase-locked amplification module, a signal processing module and the like on one circuit board, centralize the functions of a current driving module and a temperature control module of a laser on one circuit board, and are convenient for the modularization and the integration of a system; by collecting the absorption signal of the environmental gas in real time as a background curve to be deducted, a reference gas chamber is not required to be arranged in the system, so that the influence of the airtightness of the gas chamber, the adsorbability of the gas and the like on the measurement result is avoided; the system of the invention utilizes the FIR filter to extract the absorption spectral line of the gas by selecting proper filter coefficients, thereby reducing the noise interference; the 2f signal is normalized by using the direct current signal, the influence of laser light intensity jitter and the like on light intensity is eliminated, and the concentration is quickly inverted by establishing a second-order relational expression of the normalized 2f signal peak value and the concentration.
Specifically, as shown in fig. 1, it is a flowchart of the hardware system according to the embodiment of the present invention. The invention discloses a system for monitoring gas concentration, which comprises a distributed feedback laser, a signal generation module, a pyramid prism, an off-axis parabolic mirror, a photoelectric detection module, a signal transmission module, a phase-locked amplification module and a signal processing module, wherein:
laser output by the distributed feedback laser is collimated, then passes through a central hole in the off-axis parabolic mirror and a region to be detected of target gas, is reflected back to the off-axis parabolic mirror by the pyramid prism, and the returned laser is converged to a photosensitive region of the photoelectric detection module;
the signal transmission module transmits the electric signal acquired by the photoelectric detection module to the phase-locked amplification module;
the phase-locked amplification module demodulates the electric signal acquired by the photoelectric detection module by using the reference signal provided by the signal generation module;
the signal processing module extracts the 2f signal through a filter and utilizes the filter to carry out smooth filtering on the extracted signal, thereby providing the 2f signal and the direct current signal for subsequent algorithm processing.
The signal generation module generates a sawtooth signal and a sine signal, and modulates the sine signal onto the sawtooth signal as a driving signal of the distributed feedback laser;
the photoelectric detection module is used for converting the received optical signal into an electric signal containing gas concentration information to obtain a direct absorption signal.
The system also comprises a temperature control module, wherein the temperature control module is used for controlling the working temperature of the distributed feedback laser, the internal secondary temperature control can accurately control the temperature to be the required temperature, the error is +/-0.001 ℃, and the stability of the output wavelength of the distributed feedback laser is effectively ensured;
the system also comprises a current driving module, wherein the current driving module converts the modulated voltage signal into a current signal and performs current tuning on the distributed feedback laser, so that the wavelength range of laser emitted by the distributed feedback laser is ensured to completely cover the absorption wavelength of the target gas.
The system utilizes the direct current signal to carry out normalization processing on the 2f signal, a reference air chamber and a reference light path do not need to be arranged inside the system, and the complexity of the system is low.
Wherein the signal processing module extracts an absorption line of the gas by using an FIR filter.
The signal processing module is used for carrying out normalization processing on the 2f signal by using the direct current signal, and quickly inverting the concentration by establishing a second-order relation between the peak value and the concentration of the normalized 2f signal.
The inversion method comprises the following specific steps:
pre-collecting 2f signals and direct absorption signals of different concentrations in a gas area to be detected, and carrying out the following processing:
extracting the maximum value of the 2f wave crest at the position known as the absorption peak, extracting the left wing minimum value and the right wing minimum value of the 2f wave trough at the positions of two wings of the absorption peak, subtracting the average value of the left wing minimum value and the right wing minimum value from the maximum value to obtain the peak value of the 2f absorption peak, and marking as 2 fpeak;
because the number of points of the direct absorption signal is known to be equal to that of the 2f signal, selecting a certain section of direct absorption signal at the position without an absorption peak, and averaging the section of direct absorption signal to be marked as dcmean;
normalizing the peak value 2fpeak of the 2f absorption peak by using the mean value dcmean of the direct absorption signal to obtain a normalized 2f value which is marked as 2 fmean;
establishing a nonlinear relation between the concentration and 2 fmean;
and flushing the gas with the concentration to be measured into the region of the gas to be measured, calculating to obtain the normalized 2f value, and substituting the value into the established nonlinear relation to obtain the true value of the concentration of the gas to be measured.
The signal generation module, the signal transmission module, the phase-locking amplification module and the signal processing module are integrated on the same circuit board.
The temperature control module and the current driving module are integrated on the same circuit board.
Fig. 2 is a flow chart of a monitoring method according to an embodiment of the present invention. The method comprises the following steps:
the system is electrified and automatically started, the signal generation module generates waveform voltage, and the current driving module converts a voltage signal into current to drive the laser to generate a laser signal;
the laser signal passes through a target area of the gas to be detected through a central hole of the off-axis parabolic mirror, and the pyramid prism reflects the laser signal to the off-axis parabolic mirror;
the off-axis parabolic mirror converges laser on the photoelectric detection module, the photoelectric detection module converts an optical signal into a voltage signal, the signal transmission module collects the voltage signal and transmits the voltage signal to the phase-locked amplification module, and the phase-locked amplification module analyzes a 2f signal;
judging whether the light intensity meets the requirements, if not, alarming and judging whether the light intensity meets the requirements; and if the requirements are met, acquiring a 2f signal, and obtaining the gas concentration by adopting a quick inversion step.
The rapid inversion method comprises the following steps:
pre-collecting 2f signals and direct absorption signals of different concentrations in a gas area to be detected, and carrying out the following processing:
extracting the maximum value of the 2f wave crest at the position known as the absorption peak, extracting the left wing minimum value and the right wing minimum value of the 2f wave trough at the positions of two wings of the absorption peak, subtracting the average value of the left wing minimum value and the right wing minimum value from the maximum value to obtain the peak value of the 2f absorption peak, and marking as 2 fpeak;
because the number of points of the direct absorption signal is known to be equal to that of the 2f signal, selecting a certain section of direct absorption signal at the position without an absorption peak, and averaging the section of direct absorption signal to be marked as dcmean;
normalizing the peak value 2fpeak of the 2f absorption peak by using the mean value dcmean of the direct absorption signal to obtain a normalized 2f value which is marked as 2 fmean;
establishing a nonlinear relation between the concentration and 2 fmean;
and flushing the gas with the concentration to be measured into the region of the gas to be measured, calculating to obtain the normalized 2f value, and substituting the value into the established nonlinear relation to obtain the true value of the concentration of the gas to be measured.
The technical solution of the present invention is further illustrated by the following specific examples in conjunction with the accompanying drawings.
Example 1
Example 1 to monitor the change in the concentration of CO (carbon monoxide) in the air, the following example was analyzed.
Firstly, a DFB laser with the center wavelength of 2326.8nm is selected, the working temperature of the laser is set to be 29 ℃, the output wavelength of the laser is 2326.56 nm-2327.21 nm, and the absorption peak of CO at the position of 2326.823nm is completely covered.
And adjusting the light path to ensure that the voltage range of the absorbed light intensity of the detector is between 2V and 5V so as to ensure the accuracy of the measurement result.
Before measurement, the measurement concentration of CO needs to be subjected to nonlinear calibration, the maximum value of the measurement concentration is set to 1000ppm, 1000ppm is taken as the initial concentration, one point is selected at every 1000ppm, and 9 points are calibrated to obtain the relation between the absorption peak and the concentration, which is shown in figure 3;
establishing a non-linear relationship between concentration and ratio, as shown in fig. 4;
and the CO with the concentration of 2000ppm, 5000ppm and 10000ppm is measured again for verification, the measurement result is shown in the table 1, the maximum error is about 1%, the measurement precision is high, and the measurement requirement is met.
Table 1 measurement error in example 1
Example 2
Example 2 to monitor CO in air2The change in the concentration of (carbon dioxide) was analyzed in the following manner.
Selecting a DFB laser with the center wavelength of 2004nm, setting the working temperature of the laser to be 30 ℃, setting the output wavelength of the laser to be 2003.61-2004.41 nm, and completely covering CO2Absorption peak at 2004.019 nm.
And adjusting the light path to ensure that the maximum voltage value range of the absorption light intensity of the detector is between 2V and 5V so as to ensure the accuracy of the measurement result.
Before measurement, CO needs to be measured2The measured concentration is subjected to nonlinear calibration, the maximum value of the measured concentration is set to 50000ppm, 10000ppm is taken as the initial concentration, one point is selected at intervals of 10000ppm, and 5 points are calibrated in total, so that the relation between the absorption peak and the concentration is obtained and is shown in figure 5;
establishing a non-linear relationship between concentration and ratio, as shown in fig. 6;
again select CO concentrations of 10000ppm, 30000ppm and 50000ppm2The verification is carried out, the measurement result is shown in table 2, the error between the two is very small, the maximum error is within 7 percent, and the measurement requirement is met.
Table 2 measurement error in example 2
The above-mentioned embodiments are intended to illustrate the objects, technical solutions and advantages of the present invention in further detail, and it should be understood that the above-mentioned embodiments are only exemplary embodiments of the present invention and are not intended to limit the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
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| CN112504981A (en) * | 2020-11-25 | 2021-03-16 | 天津同阳科技发展有限公司 | Method for detecting motor vehicle exhaust emission by using wavelength modulation spectrum technology |
| CN113533249A (en) * | 2021-06-09 | 2021-10-22 | 山东师范大学 | A N2O detection system and method based on open optical path and wavelength modulation |
| CN114047161A (en) * | 2021-10-28 | 2022-02-15 | 中南大学 | Self-diagnosis method for laser emission wavelength |
| CN114414517A (en) * | 2021-12-17 | 2022-04-29 | 山东微感光电子有限公司 | A low-power intrinsically safe laser carbon monoxide sensing control method and system |
| CN114414517B (en) * | 2021-12-17 | 2024-02-20 | 山东微感光电子有限公司 | A low-power intrinsically safe laser carbon monoxide sensing control method and system |
| CN114993987A (en) * | 2022-05-16 | 2022-09-02 | 北京航空航天大学 | Temperature and gas concentration measurement method and system based on absorption spectrum amplitude modulation |
| CN114993987B (en) * | 2022-05-16 | 2024-04-30 | 北京航空航天大学 | Temperature and gas concentration measurement method and system based on absorption spectrum amplitude modulation |
| CN115326754A (en) * | 2022-09-14 | 2022-11-11 | 南京大学 | A temperature-free gas concentration sensor |
| CN115855875A (en) * | 2022-11-28 | 2023-03-28 | 长春理工大学 | Three-dimensional gas concentration field measurement device, method, system and supporting probe system |
| CN117747008A (en) * | 2024-02-07 | 2024-03-22 | 山东省科学院激光研究所 | A baseline fitting and noise reduction method and system for gas laser absorption spectra |
| CN117747008B (en) * | 2024-02-07 | 2024-05-03 | 山东省科学院激光研究所 | A method and system for baseline fitting and noise reduction of gas laser absorption spectrum |
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