CN211505210U - System for monitoring gas concentration in real time based on wavelength modulation spectrum technology - Google Patents
System for monitoring gas concentration in real time based on wavelength modulation spectrum technology Download PDFInfo
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Abstract
The utility model provides a system for monitoring gas concentration, includes distributed feedback laser, signal generation module, pyramid prism, off-axis parabolic mirror, photoelectric detection module, signal transmission module, lock phase amplification module and 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.
Description
Technical Field
The utility model relates to a tail gas emission field especially relates to a system of real-time supervision gas concentration based on wavelength modulation spectrum technique.
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.
SUMMERY OF THE UTILITY MODEL
In view of the above, it is a primary object of the present invention to provide a system for monitoring gas concentration, which is intended to at least partially solve at least one of the above technical problems.
In order to achieve the above object, the utility model provides a system for monitoring gas concentration, including distributed feedback laser instrument, signal generation module, pyramid prism, off-axis parabolic mirror, photodetection module, signal transmission module, lock-in amplification module and 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 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.
Based on the above technical scheme can know, the utility model discloses a system and method of monitoring gas concentration have one of following beneficial effect at least for prior art:
1. the system of the utility model collects the absorption signal of the environmental gas in real time as the background curve to be deducted, and a reference air chamber is not needed to be arranged in the system, thereby avoiding the influence of the air chamber tightness, the gas adsorptivity and the like on the measurement result;
2. the system of the utility model utilizes the FIR filter to extract the absorption spectrum line of the gas by selecting proper filter coefficient, thus reducing the noise interference;
3. the utility model utilizes the direct current signal to carry out normalization processing on the 2f signal, eliminates the influence of laser light intensity jitter, dust and the like on the light intensity, and quickly inverts the concentration by establishing a second-order linear relation between the normalized 2f signal peak value and the concentration;
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 illustrating the operation of a hardware system according to an embodiment of the present invention;
FIG. 2 is a spectrum diagram of CO absorption peak and concentration in example 1 of the present invention;
FIG. 3 is a second order nonlinear fit of CO concentration to 2fmean in example 1 of the present invention;
FIG. 4 shows CO in example 2 of the present invention2A spectrum of absorption peaks versus concentration;
FIG. 5 shows CO in example 2 of the present invention2Second order nonlinear fit plot of concentration to 2 fmean.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in detail with reference to the accompanying drawings.
The utility model discloses a system and a method for monitoring gas concentration in real time based on 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 a circuit board, centralize the functions of a current driving module and a temperature control module of a laser on a circuit board, and are convenient for the modularization and the integration of the 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 utility model utilizes the FIR filter to extract the absorption spectrum line of the gas by selecting proper filter coefficient, thus 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 work flow diagram of a hardware system according to an embodiment of the present invention. The utility model discloses a system for monitoring gas concentration, including distributed feedback laser instrument, signal generation module, pyramid prism, off-axis parabolic mirror, photoelectric detection module, signal transmission module, lock-in amplification module and 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 inversion step can be realized by adopting a known algorithm program, and the utility model discloses also does not focus on the improvement on the algorithm.
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.
The technical solution of the present invention is further explained by the following specific embodiments with reference to the attached drawings.
Example 1
Example 1 in order 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 2;
establishing a non-linear relationship between concentration and ratio, as shown in FIG. 3;
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 the calibration is carried out at 5 points in total, so that the relation between the absorption peak and the concentration is obtained and is shown in figure 4;
establishing a non-linear relationship between concentration and ratio, as shown in FIG. 5;
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, further detailed description of the objects, technical solutions and advantages of the present invention, it should be understood that the above-mentioned embodiments are only specific embodiments of the present invention, and are not intended to limit the present invention, and any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (9)
1. The utility model provides a system for monitoring gas concentration which characterized in that, includes distributed feedback laser, signal generation module, pyramid prism, off-axis parabolic mirror, photoelectric detection module, signal transmission module, lock phase amplification module and 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.
2. The system for monitoring the concentration of a gas of claim 1, wherein the signal generation module generates a sawtooth signal and a sinusoidal signal and modulates the sinusoidal signal onto the sawtooth signal as the drive signal for the distributed feedback laser.
3. The system according to claim 1, wherein the photodetection module is configured to convert the received optical signal into an electrical signal containing information on the gas concentration, resulting in a direct absorption signal.
4. The system for monitoring the concentration of the gas as claimed in claim 1, wherein the system further comprises a temperature control module, 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 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.
5. The system for monitoring the concentration of the gas as claimed in claim 1, wherein the system utilizes the direct current signal to perform normalization processing on the 2f signal, a reference gas chamber and a reference light path are not required to be arranged inside, and the complexity of the system is low.
6. The system for monitoring gas concentration of claim 1, wherein the signal processing module utilizes a FIR filter to extract absorption lines of the gas.
7. The system for monitoring the concentration of a gas according to claim 1, wherein the signal processing module normalizes the 2f signal using the dc signal and rapidly inverts the concentration by establishing a second order relation between the peak value of the normalized 2f signal and the concentration.
8. The system for monitoring the concentration of a gas of claim 1, wherein the signal generation module, the signal transmission module, the phase-locked amplification module and the signal processing module are integrated on the same circuit board.
9. The system for monitoring gas concentration of claim 4, wherein the temperature control module and the current drive module are integrated on the same circuit board.
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| CN113533249A (en) * | 2021-06-09 | 2021-10-22 | 山东师范大学 | N2O detection system and method based on open optical path and wavelength modulation |
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