CN112540059A - Ethylene detection method based on TDLAS technology - Google Patents
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- VGGSQFUCUMXWEO-UHFFFAOYSA-N Ethene Chemical compound C=C VGGSQFUCUMXWEO-UHFFFAOYSA-N 0.000 title claims abstract description 33
- 239000005977 Ethylene Substances 0.000 title claims abstract description 33
- 238000001514 detection method Methods 0.000 title claims abstract description 27
- 238000000041 tunable diode laser absorption spectroscopy Methods 0.000 title claims abstract description 17
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
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Abstract
The invention discloses an ethylene detection method based on a TDLAS technology, which comprises a system design method and a data processing method, wherein the system mainly comprises a TDLAS control panel, a temperature control module, a laser, a collimator, a beam splitter, a gas absorption cell, a standard gas module, an analog switch circuit module, a detector, a data acquisition card and an upper computer. LabVIEW software is adopted for data processing, and the data processing process comprises the following steps: scanning period alignment, error analysis and elimination, period signal averaging, data filtering and background subtraction. Compared with the prior art, the system adopts the mode of calling MATLAB in LabVIEW software to process the acquired ethylene concentration data, thereby reducing the error of an experimental result and improving the accuracy of the whole detection device.
Description
Technical Field
The invention relates to an ethylene detection method based on a TDLAS technology, which comprises a system design method and a data processing method.
Background
With the rapid development of economy in China, the living standard of people is continuously improved, the nutritional requirements on fruits and vegetables are higher and higher, and in order to ensure the fresh quality of the fruits and vegetables in the transportation process, the respiration of the fruits and vegetables is generally inhibited by properly reducing the oxygen concentration and increasing the carbon dioxide concentration, and the ethylene concentration in the refrigeration environment is controlled at the same time, so that the effect of delaying the after-ripening and aging of the fruits and vegetables is achieved.
Ethylene is a combination of ozone (O) and atmospheric chemistry3) The generated important gas can be towardsThe environment causes pollution to a certain degree; ethylene has a strong anesthetic effect on human bodies, and when a human body inhales air containing a certain amount of ethylene, consciousness loss of the human body can be caused, memory disorder is caused, and the ethylene is extremely harmful to human health; ethylene is flammable, forms an explosive mixture when mixed with air, and risks of combustion and explosion when exposed to open flames, high heat or contact with an oxidizing agent. The contact with gases such as fluorine, chlorine and the like can cause violent chemical reaction, and the gas is often used as an indicator gas for indicating spontaneous combustion of coal beds, so that ethylene also becomes one of the gases mainly detected in production and life.
However, in the process of actually measuring the ethylene concentration, the obtained data is often incomplete and inconsistent and is very easily invaded by noise, so that an error value or an abnormal value of the data is naturally generated, and meanwhile, because the data volume is too large, the data set often comes from a plurality of data sources, the initially obtained data quality generally does not meet the requirements of people, and the low-quality data can cause low-quality experimental results, so that the data processing is particularly important in the experimental process of people and is an indispensable part in the experimental process.
In conclusion, the data processing has important significance on the final experimental result in the gas detection process, the invention provides a set of detection system which is easy to operate and high in accuracy, and the data processing method provided by the invention is used for processing the acquired data, so that the detection accuracy can be greatly improved.
Disclosure of Invention
In order to overcome the prior art, the invention provides an ethylene detection method based on a TDLAS technology, which is based on the TDLAS technology and combines a tunable semiconductor laser absorption spectrum technology and uses LabVIEW software to process data to realize the detection of the ethylene concentration.
The system design mainly comprises a TDLAS control panel, a temperature control module, a laser, a collimator, a beam splitter, a gas absorption cell, a standard gas module, an analog switch circuit module, a detector, a data acquisition card and an upper computer.
The working process of the system detection system is as follows: the temperature control module coarsely adjusts the laser temperature, the signal generating circuit on the TDLAS control board generates a low-frequency scanning signal and a high-frequency modulating signal, the two paths of voltage signals are superposed and then converted into a current signal, and the working current of the laser is controlled. Laser emitted by a laser device respectively enters a gas absorption cell and a standard gas module after passing through a beam splitter, the absorption spectrum line of a detected gas is scanned, after the laser passes through the gas absorption cell and the gas standard module, an optical signal is converted into an electrical signal by a photoelectric detector, two paths of electrical signals are respectively transmitted to a TDLAS control panel by an analog switch circuit module, a secondary harmonic signal is detected after the laser is processed by a preamplifier and a lock-in amplifier, finally, the data is collected by a data collection card and then transmitted to an upper computer, and then, LabVIEW software is used for processing the data, and finally, the detection result of the gas concentration is obtained.
LabVIEW software is adopted for data processing, and the data processing process comprises the following steps: scanning period alignment, error analysis and elimination, period signal averaging, data filtering and background subtraction;
scanning period alignment: during data acquisition, a second harmonic signal acquired by an acquisition card is not a single-cycle signal but a continuous signal with a plurality of scanning cycles, the data acquisition card simultaneously acquires a gas second harmonic signal and a sawtooth wave scanning signal, the sawtooth wave scanning signal is used as a cycle alignment signal, one cycle of the sawtooth wave corresponds to one cycle of the second harmonic wave, the starting point of one cycle of the second harmonic wave can be determined by searching the lowest point of the sawtooth wave, the cycle signals are rearranged according to a fixed length, and the alignment signal and the second harmonic wave signal are respectively stored in a matrix for subsequent processing;
error analysis and elimination: in the process of scanning an ethylene gas absorption spectral line by a sawtooth wave, a sawtooth wave waveform in a period is selected, the sawtooth wave frequency is 5Hz, the peak value is 300mV, the number of sampling points is 600, the sawtooth wave waveform between 300 plus 900 sampling points is observed, about 20 sampling points at the head and the tail vibrate, the vibrating sampling points can cause fluctuation to the output wavelength of a laser, the same absorption spectral line of the sawtooth wave repeatedly scanning gas is caused, the detected harmonic signal can generate distortion, and the detection precision of a system is influenced, so that in subsequent processing, the sampling frequency is 3.2kS/s, and 20 points vibrating at the head and the tail are removed as data for data processing.
Periodic signal averaging: the periodic signal averaging needs to obtain an average value of signals of multiple periods, so that the interference of random noise to a single-period signal is reduced, the period number is reasonably selected, the increase of operation time caused by excessive period number is avoided, and the poor data processing effect caused by too few period number is avoided. The method utilizes matrix operation to average periodic signals of original data, selects 30 periods and obtains ideal signals.
Data filtering: by analyzing the second harmonic signal of the ethylene gas, it was found that there was noise interference at the second harmonic. According to the analysis of the fourth chapter, the data processing is performed by adopting two filtering methods, namely Kalman filtering and singular value decomposition, so that the two filtering methods have approximately the same effect, and compared with the Kalman filtering, the singular value decomposition method has a smoother signal.
Background subtraction: in gas concentration detection systems, there are still various types of background signal disturbances in the detected harmonic signals, including residual amplitude modulation noise and large-pitch optical interference fringes. The residual amplitude modulation comprises a linear part and a nonlinear part, wherein the linear part is caused by the linear relation between the light intensity of the laser and the injected current, the nonlinear part is caused by slight nonlinearity of the signal amplitude in a laser driver or a signal source, and particularly, when the sawtooth wave of a scanning signal contains small stripes, a periodic background signal appears on the harmonic wave; the amplitude of the large-spacing optical interference fringes changes slowly, and the two kinds of noise cannot be filtered by a filtering algorithm and can be filtered by a background subtraction method. When the sawtooth wave of the scanning signal contains small stripes, a periodic background signal appears on the harmonic wave; the amplitude of the large-spacing optical interference fringes changes slowly, and the two kinds of noise cannot be filtered by a filtering algorithm and can be filtered by a background subtraction method.
Drawings
FIG. 1 is a block diagram of an ethylene concentration detection system;
FIG. 2 LabVIEW monitoring software;
FIG. 3 LabVIEW software flow chart;
FIG. 4 is a second harmonic signal of one period;
FIG. 5 is a sawtooth waveform;
fig. 6 shows the lowest point and the highest point of the sawtooth wave: (a) lowest point, (b) highest point;
FIG. 7 actual detection signals;
FIG. 8 is a singular value filtered signal;
FIG. 9 before and after second harmonic background subtraction;
FIG. 10 second harmonic after background subtraction;
fig. 11 background subtraction: (a) before background deduction, (b) after background deduction;
FIG. 12 least squares fit results before background subtraction;
FIG. 13 least squares fit results after background subtraction;
Detailed Description
The present invention will be described in detail below with reference to the drawings and specific embodiments, but the scope of the present invention is not limited thereto.
The structure of an ethylene concentration detection system based on the TDLAS technology is shown in figure 1.
The whole gas concentration detection system mainly comprises a TDLAS control panel, a temperature control module, a laser, a collimator, a beam splitter, a gas absorption cell, a standard gas module, an analog switch circuit module, a detector, a data acquisition card and an upper computer.
The data processing of the ethylene concentration detection system adopts LabVIEW software, the LabVIEW software has powerful functions of data acquisition, data processing, data analysis and instrument control, and simultaneously has a good interface, the data acquisition of an acquisition card can be realized by compiling an acquisition program through the LabVIEW software, and the LabVIEW detection interface is shown in figure 2.
Turning on a starting switch, starting a monitoring interface, and indicating that the modules are communicated when a green light is on; the parameter setting comprises temperature, laser current, current upper limit and photoelectric current setting, and when the actual value is larger than the set value, the corresponding thermistor open circuit, laser open circuit, refrigeration open circuit and current overrun can correspondingly light up red light; the right side interface mainly comprises zero correction, linear correction, parameter setting and sampling rate setting, the gas absorption spectral line comprises display of an initial signal, a single-period signal, concentration-time and gas concentration, and the gas signal can be observed in a corresponding module by turning on a sampling switch.
LabVIEW can provide a good interactive design platform in the aspect of signal acquisition, but a tool kit provided in the aspect of operational analysis is insufficient, so that the operational development efficiency of LabVIEW is limited. MATLAB contains numerous specialized tool boxes, can provide the instrument support that the professional function is abundant and use is simple convenient for each field, has the advantage of high programming efficiency, calls MATLAB data processing program in LabVIEW software, realizes that the two advantage complements each other. Fig. 3 shows a system software flowchart after LabVIEW software is used in combination with system parameter setting and hardware circuit monitoring, and the data processing process in the flowchart mainly includes: scanning period alignment, error analysis and elimination, periodic signal averaging, data filtering and background subtraction.
During data acquisition, a second harmonic signal acquired by the acquisition card is not a single-cycle signal but a continuous signal with a plurality of scanning cycles, the data acquisition card simultaneously acquires a gas second harmonic signal and a sawtooth wave scanning signal, the sawtooth wave scanning signal is used as a cycle alignment signal, one cycle of the sawtooth wave corresponds to one cycle of the second harmonic, and the starting point of one cycle of the second harmonic can be determined by searching the lowest point of the sawtooth wave, as shown in fig. 4.
The frequency of the sawtooth waves is 5Hz, the sampling frequency of the acquisition card is 3kS/s, the data volume is considered, the subsequent data processing requirements are met, and 600 sampling points are obtained in each period. Therefore, the initial point of the first period is found, the periodic signals are rearranged according to the fixed length, and the alignment signals and the second harmonic signals are respectively stored in the matrix for subsequent processing.
In the process of scanning the ethylene gas absorption line by the sawtooth wave, a sawtooth wave shape in one period is selected, as shown in fig. 5. The sawtooth wave frequency is 5Hz, the peak-to-peak value is 300mV, the number of sampling points is 600, the sawtooth wave waveform between 300 sampling points and 900 sampling points is observed, the lowest point part and the highest point part are shown in figure 6, about 20 sampling points at the head and the tail vibrate, the vibrating sampling points can cause fluctuation to the output wavelength of a laser, the sawtooth wave repeatedly scans the same absorption spectral line of gas, the detected harmonic signal can be distorted, and the detection precision of a system is influenced, so in the subsequent processing, the sampling frequency is 3.2kS/s, and 20 points at the head and the tail which vibrate are removed as data for data processing.
Gross error refers to a significant deviation between a measured value and an actual value, and may be caused by various reasons such as sudden voltage change, instrument failure, electromagnetic wave interference and the like. The gross error is not compensated, exists in all scientific experiments, can not be completely eliminated, and can only be reduced to a certain extent. As an outlier of the severe distortion reality, it deviates 3 times the standard deviation and should therefore be eliminated during data processing, otherwise the standard deviation and mean deviation would be severely affected.
The periodic signal averaging needs to obtain an average value of signals of multiple periods, so that the interference of random noise to a single-period signal is reduced, the period number is reasonably selected, the increase of operation time caused by excessive period number is avoided, and the poor data processing effect caused by too few period number is avoided. The method utilizes matrix operation to average periodic signals of original data, selects 30 periods and obtains ideal signals.
Noise exists certainly in the gaseous detecting system based on TDLAS technique, can cause the interference to gas absorption spectral line moreover, influences the detection precision.
The second harmonic signal of the ethylene gas was analyzed and it was found that there was noise interference at the second harmonic as shown in fig. 7. According to the noise analysis, a singular value decomposition filtering method is adopted for data processing, and signals after singular value filtering are shown in fig. 8.
In gas concentration detection systems, there are still various types of background signal disturbances in the detected harmonic signals, including residual amplitude modulation noise and large-pitch optical interference fringes. The residual amplitude modulation comprises a linear part and a nonlinear part, wherein the linear part is caused by the linear relation between the light intensity of the laser and the injected current, the nonlinear part is caused by slight nonlinearity of the signal amplitude in a laser driver or a signal source, and particularly, when the sawtooth wave of a scanning signal contains small stripes, a periodic background signal appears on the harmonic wave; the amplitude of the large-spacing optical interference fringes changes slowly, and the two kinds of noise cannot be filtered by a filtering algorithm, and can be filtered by a background subtraction method, wherein the filtering process is shown in fig. 9. The subtracted background signal, which is the subtraction of the original and background signals in fig. 9, facilitates the extraction of the characteristics of the signal therefrom for the purpose of gas concentration inversion.
The concentration of ethylene is 30ppm, the ethylene and the nitrogen are mixed and divided into 20% -100% in equal proportion by using a gas divider, the obtained second harmonic signal is shown in fig. 10, and as can be seen from fig. 10, after data processing, a signal characteristic value is easily extracted from the second harmonic signal in the graph, so that concentration inversion is further realized.
The least squares method is used to invert the rationale for gas concentration: and performing least square fitting on the to-be-detected signal with unknown concentration according to the reference signal standard harmonic spectrum with known concentration to obtain the concentration value of the to-be-detected gas. The gas concentration of the ethylene gas standard block of 50ppm was chosen as the reference signal. Fig. 11 shows the second harmonic signals before and after background subtraction.
The least square fitting is performed on the reference signal before background subtraction and the signal to be measured, the result is shown in fig. 12, the abscissa is the amplitude of the reference signal, the ordinate is the amplitude of the signal to be measured, and the fitting result of the least square method before background subtraction is: the curve fitting degree is 0.9884, the concentration ratio is 0.0021179, and the concentration of the gas to be detected is 29.9 ppm; the coincidence of the fitted straight line is not good, and an error is caused to a concentration result; after the background is deducted, as shown in fig. 13, the curve fitting degree is 0.9959, the concentration ratio is 0.00036611, the concentration of the gas to be measured is 22.5ppm, compared with the gas before the background is eliminated, the curve fitting degree is improved, and the calculation result is in line with the reality.
The invention realizes the collection and processing of ethylene gas concentration data by using LabVIEW system software; establishing a data processing flow of scanning period alignment, error analysis and elimination, period signal averaging, data filtering and background subtraction, and detecting the multi-concentration ethylene gas by using the processing flow; and the ethylene with the concentration of 50ppm is used as a reference signal to detect unknown ethylene, so that the concentration inversion of ethylene gas is realized.
It should be emphasized that the embodiments described herein are illustrative rather than restrictive, and thus the present invention is not limited to the embodiments described in the detailed description, but also includes other embodiments that can be derived from the technical solutions of the present invention by those skilled in the art.
Claims (2)
1. An ethylene detection method based on TDLAS technology is characterized by comprising system design and data processing, wherein:
the system design mainly comprises a TDLAS control panel, a temperature control module, a laser, a collimator, a beam splitter, a gas absorption cell, a standard gas module, an analog switch circuit module, a detector, a data acquisition card and an upper computer;
the temperature control module coarsely adjusts the laser temperature, a signal generating circuit on the TDLAS control board generates a low-frequency scanning signal and a high-frequency modulating signal, two paths of voltage signals are superposed and then converted into a current signal, and the working current of the laser is controlled; laser emitted by a laser device respectively enters a gas absorption cell and a standard gas module after passing through a beam splitter, the absorption spectrum line of a detected gas is scanned, after the laser passes through the gas absorption cell and the gas standard module, an optical signal is converted into an electrical signal by a photoelectric detector, two paths of electrical signals are respectively transmitted to a TDLAS control panel by an analog switch circuit module, a secondary harmonic signal is detected after the laser is processed by a preamplifier and a lock-in amplifier, finally, the data is collected by a data collection card and then transmitted to an upper computer, and then, LabVIEW software is used for processing the data, and finally, the detection result of the gas concentration is obtained.
2. Data processing according to claim 1, characterized in that the data processing method comprises the following steps:
LabVIEW software is adopted for data processing, and the data processing process comprises the following steps: scanning period alignment, error analysis and elimination, period signal averaging, data filtering and background subtraction;
step 1, scanning periods are aligned, when data are collected, a second harmonic signal obtained by a collecting card is not a single-period signal but a continuous signal of a plurality of scanning periods, the data collecting card simultaneously collects a gas second harmonic signal and a sawtooth wave scanning signal, the sawtooth wave scanning signal is used as a period alignment signal, one period of the sawtooth wave corresponds to one period of the second harmonic, the starting point of one period of the second harmonic can be determined by searching the lowest point of the sawtooth wave, signals of each period are rearranged according to a fixed length, and the alignment signal and the second harmonic signal are respectively stored in a matrix for subsequent processing;
step 2, error analysis and elimination, wherein in the process of scanning an ethylene gas absorption spectral line by a sawtooth wave, a sawtooth wave waveform in a period is selected, the sawtooth wave frequency is 5Hz, the peak value is 300mV, the number of sampling points is 600, the sawtooth wave waveform between 300 and 900 sampling points is observed, about 20 sampling points at the head and the tail vibrate, the vibrating sampling points can cause fluctuation to the output wavelength of a laser, the same absorption spectral line of the gas is repeatedly scanned by the sawtooth wave, the detected harmonic signal can be distorted, and the detection precision of the system is influenced, so that in subsequent processing, the sampling frequency is 3.2kS/s, and 20 points vibrating at the head and the tail are removed as data for data processing;
and step 3, averaging the periodic signals, wherein the averaging of the periodic signals needs to obtain an average value of signals of a plurality of periods, so that the interference of random noise to a single-period signal is reduced, the period number is reasonably selected, the increase of operation time caused by excessive period number is avoided, and the poor data processing effect caused by insufficient period number is avoided. The method comprises the steps of averaging periodic signals of original data by utilizing matrix operation, and selecting 30 periods to obtain ideal signals;
and 4, data filtering, and finding that noise interference exists in the second harmonic by analyzing the second harmonic signal of the ethylene gas. According to the analysis of the fourth chapter, two filtering methods of Kalman filtering and singular value decomposition are respectively adopted for data processing, so that the two filtering methods have approximately the same effect, and compared with the Kalman filtering, the singular value decomposition method has smoother signals;
step 5, background subtraction, in the gas concentration detection system, the detected harmonic signals still have various interference of background signals, including residual amplitude modulation noise and large-interval optical interference fringes, the residual amplitude modulation includes linear and nonlinear parts, the linear part is caused by the linear relation between the laser light intensity and the injection current, the nonlinear part is caused by the slight nonlinearity of the signal amplitude in a laser driver or a signal source, and particularly when the sawtooth wave of the scanning signal contains small fringes, periodic background signals appear on the harmonic waves; the amplitude of the large-spacing optical interference fringes changes slowly, and the two kinds of noise cannot be filtered by a filtering algorithm and can be filtered by a background subtraction method.
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Cited By (2)
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| CN113758899A (en) * | 2021-11-11 | 2021-12-07 | 国网湖北省电力有限公司检修公司 | Micro-water measuring method and device based on TDLAS technology |
| CN114460023A (en) * | 2022-04-14 | 2022-05-10 | 华电智控（北京）技术有限公司 | Detection method, system and device for simultaneously measuring concentrations of multiple gases |
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