Variable optical path laser gas detection method and system

Abstract

The invention provides a variable optical path laser gas detection method and system, the method is used for single gas component full-scale gas detection in a laser gas sensor, and the specific method is as follows: a photoelectric detector is arranged at each of different optical path positions of a detection optical path of a multi-reflection optical cavity; when the concentration of the gas to be detected is preset to be small, the photoelectric detector at the maximum optical path position is firstly switched to be the current detection photoelectric detector, and if the output signal of the current detection photoelectric detector does not meet the switching threshold condition, the output signal of the current detection photoelectric detector is used for calculating a gas detection result; otherwise, the photoelectric detector at the next small optical path position is switched to the current detection photoelectric detector for continuous judgment; the condition of the switching threshold is that the minimum AD value of an electric signal absorption area converted by the current photoelectric detector is 0, and V2f is greater than V1f and greater than V3f; v1f, V2f and V3f are the first harmonic amplitude, the second harmonic amplitude and the third harmonic amplitude calculated by the FFT respectively.

Description

Technical Field

[0001] The present invention relates to the technical field of laser gas detection, and specifically, to a variable optical path laser gas detection method and system. Background Art

[0002] A laser gas sensor refers to a sensor that uses laser spectroscopy technology to detect the concentration of a target gas in the environment, mainly including a laser, a multiple reflection optical cavity, a photodetector, and a signal processing circuit for processing and analyzing the electrical signal output by the photodetector, etc.

[0003] The core problems faced in the design of single-component laser gas sensors are: (1) The relationship between the optical path and the detection limit The detection limit is mainly determined by the sensitivity of the sensor, and the sensitivity is proportional to the optical path length. According to the Beer-Lambert Law, the longer the optical path, the more sufficient the interaction between the laser and the target gas molecules, the stronger the absorption signal, and thus the lower concentration of gas can be detected.

[0004] (2) The saturation problem of the optical path in the detection of high-concentration gases When the gas concentration is relatively high, the laser will be over-absorbed in the long optical path, resulting in signal saturation or non-linear response. This phenomenon will limit the accuracy of the sensor in the detection of high-concentration gases.

[0005] Therefore, in the actual application design, if the designed optical path is long, although the detection limit can be guaranteed, there will be a high-concentration saturation problem; if the designed optical path is short, although high-concentration target gases can be detected, the detection limit cannot be guaranteed.

[0006] Therefore, how to achieve full-range detection of a single-component target gas in a single optical system and meet the optical path requirements of multiple gases is the main challenge in the design of current laser gas sensors. Summary of the Invention

[0007] Based on this, it is necessary to provide a laser multi-gas micro-leakage detection method, system and medium for the above technical problems.

[0008] To achieve the above object, the first aspect of the present invention provides a variable optical path laser gas detection method for full-range gas detection of a single gas component in a laser gas sensor. The method is as follows: A photodetector is respectively arranged at different optical path positions of the detection optical path of the multiple reflection optical cavity; If the preset gas concentration to be detected is low, first switch the photodetector at the maximum optical path position to the current detection photodetector. If the output signal of the current detection photodetector does not meet the switching threshold condition, use the output signal of the current detection photodetector to calculate the gas detection result; otherwise, switch the photodetector at the next smaller optical path position to the current detection photodetector and continue to judge; The switching threshold condition is: The minimum AD value of the absorption region of the electrical signal converted by the current photodetector is 0, and it meets V 1f >V 2f >V 3f ; Among them, V 1f 、V 2f 、V 3f are respectively the amplitudes of the first harmonic, the second harmonic, and the third harmonic calculated after performing FFT calculation on the waveform of the electrical signal converted by the current photodetector.

[0009] Based on the above, the method for obtaining the minimum AD value of the absorption region of the electrical signal converted by the current photodetector is: The method for obtaining the minimum AD value of the absorption region of the electrical signal converted by the current photodetector is: Take the output signal wave of the current photodetector in one detection period. Assume the total number of sampling points is M, and the AD values of these M sampling points are stored in data1; Group the output signal wave with N sampling points as a group; Let the AD value of the first sampling point in each group be a1, and the AD value of the (N - 1)-th sampling point be b1; the AD value of the second sampling point in each group be a2, and the AD value of the N-th sampling point be b2; Calculate sum1 = (a1 + b1)N / 2; Calculate sum2 = the sum after adding the AD values starting from the AD value of the first sampling point and adding up to the AD value of the (N - 1)-th sampling point; Subtract sum1 from sum2, take the absolute value, and then store it in data; Calculate sum1’ = (a1 + b1)N / 2; Calculate sum2’ = the sum after adding the AD values starting from the AD value of the second sampling point and adding up to the AD value of the N-th sampling point; Subtract sum1’ from sum2’, take the absolute value, and then store it in data; Calculate the absolute values of the differences between sum1 and sum2, and between sum1’ and sum2’ for each group respectively, and then store them in data respectively; Find the maximum value of the data "data" and remember the position of the maximum value in the data "data". Take the AD value at the corresponding position in the data "data1" according to the position of the found maximum value in the data "data", which is the minimum AD value.

[0010] Based on the above, the method for calculating the gas detection result using the output signal of the current detection photodetector is as follows: Calculate the fundamental harmonic amplitude V1f of the non-absorption region of the output signal waveform of the current detection photodetector using FFT, and then calculate the second harmonic amplitude V2f of the absorption region of the output signal waveform or the entire output signal waveform using the lock-in amplification algorithm, and obtain the absorption coefficient ε = V2f / V1f of the current detection photodetector. Output the gas concentration according to the absorption coefficient ε and the calibrated parameters, that is, the gas detection result.

[0011] The second aspect of the present invention provides a variable optical path laser gas detection method for full-range gas detection of a single gas component in a laser gas sensor. The method is as follows: A photodetector is respectively arranged at different optical path positions of the detection optical path of the multi-reflection optical cavity. Preset that the gas concentration to be detected is large. First, switch the photodetector at the minimum optical path position to the current detection photodetector. If the output signal of the current detection photodetector does not meet the switching threshold condition, calculate the gas detection result using the output signal of the current detection photodetector; otherwise, switch the photodetector at the next larger optical path position to the current detection photodetector and continue to judge. The switching threshold condition is: The minimum AD value of the absorption region of the electrical signal converted by the current photodetector is greater than 0, and the absorption coefficient ε = 0. Wherein, the absorption coefficient ε = V2f / V1f, V1f is the fundamental harmonic amplitude of the non-absorption region of the output signal waveform of the current detection photodetector calculated by FFT, and V2f is the second harmonic amplitude of the absorption region of the output signal waveform or the entire output signal waveform calculated by the lock-in amplification algorithm.

[0012] The third aspect of the present invention provides a variable optical path harmonic laser gas detection system, including a multi-reflection optical cavity, a laser, a photodetector, and a processor connected to the laser through a drive circuit and connected to the photodetector through a signal processing circuit. The photodetectors are multiple and are placed at different optical path positions of the detection optical path of the multi-reflection optical cavity. When performing gas detection, the variable optical path laser gas detection method described above is adopted.

[0013] The present invention has prominent substantial features and significant progress compared with the prior art. Specifically: The present invention can achieve full-range detection of a single-component target gas by placing a photodetector at different optical paths of a multiple-reflection optical cavity and performing detection by switching the photodetectors at different optical paths.

[0014] The method provided by the present invention for obtaining the minimum AD value of the electrical signal absorption region converted by the current photodetector can quickly and accurately find the minimum AD value to switch to a suitable current detection photodetector for gas concentration detection. Description of the Drawings

[0015] Figure 1 It is a diagram showing the placement of photodetectors at different optical paths of the present invention.

[0016] Figure 2 It is a diagram of the single-component full-range gas detection method in Embodiment 1.

[0017] Figure 3 It is a diagram of the single-component full-range gas detection method in Embodiment 2.

[0018] Figure 4 It is a schematic diagram of the system structure in Embodiment 3. Detailed Embodiments

[0019] The technical solutions of the present invention will be further described in detail below through specific embodiments.

[0020] Some technical features involved in the present invention are described as follows: AD value: refers to the AD value output by the signal processing circuit in the laser gas sensor.

[0021] Absorption region: is the region where the laser wavelength matches the absorption line of the target gas, and gas molecules will absorb the laser of a specific wavelength. Non-absorption region: is the region where the laser wavelength is not absorbed by the target gas. Generally, the signal to be analyzed obtained within a detection period T can be divided into a non-absorption region and an absorption region.

[0022] Embodiment 1 As Figure 1 and Figure 2 shown, this embodiment provides a variable optical path laser gas detection method for full-range gas detection of a single gas component in a laser gas sensor. Among them, a photodetector is respectively arranged at different optical path positions of the detection optical path of the multiple-reflection optical cavity, i = 1, 2,..., n, and the smaller the i value, the greater the optical path; in this embodiment, the preset gas concentration to be detected is small, and the lower limit of detection is ensured first. i Specifically, the detection method is as follows:

[0023] First, perform parameter initialization; First, perform parameter initialization; Then a laser driver signal is generated, and the laser emits detection laser to start detection; The processor processes the output electrical signal of the current detected photodetector, that is, the PN i output electrical signal of the photodetector at the position, i = 1, 2, …, n; Obtain the PN i minimum AD value AD of the electrical signal absorption region converted by the photodetector at the position i , and at the same time, perform FFT calculation on the waveform of the electrical signal converted by the photodetector at the PN i position, and calculate the fundamental harmonic amplitude V 1f , second harmonic amplitude V 2f and third harmonic amplitude V 3f ; If the minimum AD value AD of the electrical signal absorption region converted by the photodetector at the PN i position is not 0 and does not meet V i > V 1f > V 2f > V 3f , then use the output signal of the photodetector at the PN i position to calculate the gas detection result; otherwise, let i = i + 1, and switch the current detected photodetector to the PN i position photodetector to continue the judgment; Switch in sequence until the appropriate photodetector is switched to output the gas detection result.

[0024] In some exemplary embodiments, the method for obtaining the minimum AD value of the electrical signal absorption region converted by the current photodetector is as follows: Take the output signal wave of the current photodetector in a detection period, assume the total number of sampling points is M, and store the AD values of these M sampling points in the data data1; Group the output signal wave with N sampling points as a group; Assume that the AD value of the first sampling point in each group is a1, and the AD value of the N - 1th sampling point is b1; the AD value of the second sampling point in each group is a2, and the AD value of the Nth sampling point is b2; Calculate sum1 = (a1 + b1)N / 2; Calculate sum2 = the sum after adding the AD values starting from the first sampling point and adding up to the AD value of the N - 1th sampling point; Subtract sum1 from sum2, take the absolute value, and then store it in the data data; Calculate sum1' = (a1 + b1)N / 2; Calculate sum2' = the sum after adding the AD values starting from the second sampling point and adding up to the AD value of the Nth sampling point; Take the absolute value after subtracting sum1' from sum2', and then store it in the data data; Calculate the absolute values after subtracting sum1 from sum2 and sum1' from sum2' for each group respectively, and then store them in the data data respectively; Find the maximum value in the data data, and remember the position of the maximum value in the data data; Take the AD value at the corresponding position in the data data1 according to the position of the found maximum value in the data data, which is the minimum AD value.

[0025] In some exemplary embodiments, the method for calculating the gas detection result using the output signal of the current detection photodetector is as follows: Use FFT to calculate the fundamental harmonic amplitude V1f of the non-absorption region of the output signal waveform of the current detection photodetector, and then use the lock-in amplification algorithm to calculate the second harmonic amplitude V2f of the absorption region of the output signal waveform or the entire output signal waveform, and obtain the absorption coefficient ε = V2f / V1f of the current detection photodetector; According to the absorption coefficient ε and the calibrated parameters, the gas concentration can be output, that is, the gas detection result.

[0026] It should be noted that: According to the Beer-Lambert law, the absorption of light is proportional to the gas concentration and the optical path length: A = ε·c· l Where: A is the absorbance, ε is the absorption coefficient (unit: L·mol⁻¹·cm⁻¹), c is the gas concentration (unit: mol / L), l is the optical path length (unit: cm); Then the gas concentration c = A / (ε· l ); By pre-calibrating the parameters of the absorbance A and the optical path length l , then the output gas concentration c can be obtained according to the absorption coefficient ε, that is, the gas detection result.

[0027] It should be noted that in this embodiment, after each gas detection result is obtained and the detection is completed, the photodetector at the default maximum optical path position is switched back.

[0028] Embodiment 2 As Figure 3 shown, this embodiment provides a variable optical path laser gas detection method for full-range gas detection of a single gas component in a laser gas sensor. Among them, the PN at different optical path positions of the detection optical path of the multi-reflection optical cavity iOne photodetector is respectively provided, where \(i = 1, 2, \ldots, n\). The smaller the value of \(i\), the greater the optical path. In this embodiment, the gas concentration to be detected is preset to be large to ensure that the gas is highly concentrated and unsaturated.

[0029] The specific detection method is as follows: First, perform parameter initialization; Then, generate a laser driver signal, and the laser emits detection laser to start detection; The processor processes the output electrical signal of the current detection photodetector, that is, the output electrical signal of the photodetector at the PN i position, where \(i = n, n - 1, \ldots, 2, 1\); Obtain the minimum AD value \(AD\) of the absorption region of the electrical signal converted by the photodetector at the PN i position, and at the same time, for the waveform of the electrical signal converted by the photodetector at the PN i position, use FFT to calculate the fundamental harmonic amplitude \(V1f\) of the non - absorption region, and then use the lock - in amplification algorithm to calculate the second - harmonic amplitude \(V2f\) of the absorption region of the output signal waveform or the entire output signal waveform, and obtain the absorption coefficient \(\varepsilon\) of the detection photodetector at the PN i position i \(=\frac{V2f}{V1f}\) i If the minimum AD value \(AD\) of the absorption region of the electrical signal converted by the photodetector at the PN i position is greater than 0, and \(\varepsilon\) i \( = 0\), then use the output signal of the photodetector at the PN i position to calculate the gas detection result; otherwise, let \(i = i - 1\), and switch the current detection photodetector to the photodetector at the PN i position to continue the judgment; i Switch in sequence until the appropriate photodetector is switched to output the gas detection result.

[0030] In some exemplary embodiments, the method for obtaining the minimum AD value of the absorption region of the electrical signal converted by the current photodetector is as follows: Take the output signal wave of the current photodetector in one detection period. Assume that the total number of sampling points is \(M\), and the AD values of these \(M\) sampling points are stored in the data data1; Group the output signal wave with \(N\) sampling points as a group; Let the AD value of the first sampling point in each group be \(a1\), and the AD value of the \((N - 1)\) - th sampling point be \(b1\); the AD value of the second sampling point in each group be \(a2\), and the AD value of the \(N\) - th sampling point be \(b2\); Calculate \(sum1=(a1 + b1)\frac{N}{2}\); ​​​​​Calculate sum2 = the sum obtained by successively adding the AD values from the first sampling point to the AD value of the (N - 1)-th sampling point; Subtract sum1 from sum2, take the absolute value, and then store it in the data data; Calculate sum1' = (a1 + b1)N / 2; Calculate sum2' = the sum obtained by successively adding the AD values from the second sampling point to the AD value of the N-th sampling point; Subtract sum1' from sum2', take the absolute value, and then store it in the data data; Calculate the absolute values of the differences between sum1 and sum2 for each group respectively, and the absolute values of the differences between sum1' and sum2' for each group respectively, and then store them in the data data respectively; Find the maximum value of the data data and remember the position of the maximum value in the data data; Take the AD value at the corresponding position in the data data1 according to the position of the found maximum value in the data data, which is the minimum AD value.

[0031] In some exemplary embodiments, the method for calculating the gas detection result using the output signal of the current detection photodetector is as follows: According to the absorption coefficient ε and the calibrated parameters, the gas concentration can be output, that is, the gas detection result.

[0032] It should be noted that: According to the Beer-Lambert law, the absorption of light is proportional to the gas concentration and the optical path length: A = ε·c· l Where: A is the absorbance, ε is the absorption coefficient (unit: L·mol⁻¹·cm⁻¹), c is the gas concentration (unit: mol / L), l is the optical path length (unit: cm); Then the gas concentration c = A / (ε· l ) By pre-calibrating the parameters of the absorbance A and the optical path length l , then the output gas concentration c can be obtained according to the absorption coefficient ε, that is, the gas detection result.

[0033] It should be noted that in this embodiment, after each gas detection result is obtained and the detection is completed, it switches back to the photodetector at the default minimum optical path position.

[0034] Embodiment 3 This embodiment provides a variable optical path harmonic laser gas detection system, including a multi-reflection optical cavity, a laser, a photodetector, and a processor connected to the laser through a drive circuit and connected to the photodetector through a signal processing circuit; there are multiple photodetectors, which are placed at different optical path positions of the detection optical path of the multi-reflection optical cavity, such as Figure 4 shown; When performing gas detection, the variable optical path laser gas detection method described in Embodiment 1 or Embodiment 2 can be adopted.

[0035] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them; although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that: it is still possible to modify the specific implementation manners of the present invention or perform equivalent replacements for some technical features; without departing from the spirit of the technical solutions of the present invention, they should all be covered within the scope of the technical solutions claimed by the present invention.

Claims

1. A variable optical path laser gas detection method, characterized in that: For full-range gas detection of single gas component in laser gas sensor, the method is: A photoelectric detector is respectively arranged at different optical path positions of the detection light path of the multi-reflection optical cavity; The concentration of the gas to be detected is preset to be small, and the photoelectric detector at the maximum optical path position is first switched to the current detection photoelectric detector. If the output signal of the current detection photoelectric detector does not meet the switching threshold condition, the output signal of the current detection photoelectric detector is used to calculate the gas detection result; otherwise, the photoelectric detector at the next small optical path position is switched to the current detection photoelectric detector to continue the judgment; The switching threshold conditions are: The minimum AD value of the electrical signal absorption area converted by the current photodetector is 0 and meets V 1f >V 2f >V 3f ; Among them, V 1f 、V 2f 、V 3f They are the first harmonic amplitude, the second harmonic amplitude and the third harmonic amplitude calculated after performing FFT calculation on the electrical signal waveform converted by the current photodetector.

2. The variable optical path laser gas detection method according to claim 1, characterized in that: The method for obtaining the minimum AD value of the electrical signal absorption area converted by the current photodetector is: Take the output signal wave of the current photoelectric detector within one detection cycle, set the total number of sampling points to M, and store the AD values ​​of the M sampling points in data1; The output signal wave is grouped into N sampling points; Suppose the AD value of the first sampling point in each group is a1, and the AD value of the N-1th sampling point is b1; the AD value of the second sampling point in each group is a2, and the AD value of the Nth sampling point is b2; Calculate sum1 = (a1 + b1) N / 2; Calculate sum2 = add the AD values ​​of the first sampling point in sequence to the sum of the AD values ​​of the N-1th sampling point; Subtract sum1 from sum2, take the absolute value, and store it in data; Calculate sum1'=(a1+b1)N / 2; Calculate sum2' = the sum of the AD values ​​starting from the second sampling point and added to the AD value of the Nth sampling point; Subtract sum1' from sum2', take the absolute value, and store it in data; Calculate the absolute value of sum1 and sum2 of each group, as well as the absolute value of sum1' and sum2', and store them in data respectively; Find the maximum value of data data and remember the position of the maximum value in data data; According to the position of the maximum value found in data data, the AD value of the corresponding position in data data1 is taken as the minimum AD value.

3. The variable optical path laser gas detection method according to claim 1, characterized in that: The method for calculating the gas detection result using the output signal of the current detection photoelectric detector is: Use FFT to calculate the first harmonic amplitude V1f of the non-absorption area of ​​the output signal waveform of the current detection photodetector, and then use the phase-locked amplification algorithm to calculate the absorption area of ​​the output signal waveform or the second harmonic amplitude V2f of the entire output signal waveform to obtain the absorption coefficient ε=V2f / V1f of the current detection photodetector; The gas concentration, i.e. the gas detection result, can be output based on the absorption coefficient ε and the calibrated parameters.

4. A variable optical path laser gas detection method, characterized in that: For full-range gas detection of single gas component in laser gas sensor, the method is: A photoelectric detector is respectively arranged at different optical path positions of the detection light path of the multi-reflection optical cavity; If the concentration of the gas to be detected is large, the photoelectric detector at the minimum optical path position is first switched to the current detection photoelectric detector. If the output signal of the current detection photoelectric detector does not meet the switching threshold condition, the output signal of the current detection photoelectric detector is used to calculate the gas detection result; otherwise, the photoelectric detector at the next large optical path position is switched to the current detection photoelectric detector to continue the judgment; The switching threshold conditions are: The minimum AD value of the electrical signal absorption zone converted by the current photodetector is greater than 0, and the absorption coefficient ε=0; Among them, the absorption coefficient ε=V2f / V1f, V1f is the first harmonic amplitude of the non-absorption area of ​​the output signal waveform of the current detection photodetector calculated by FFT, and V2f is the absorption area of ​​the output signal waveform or the second harmonic amplitude of the entire output signal waveform calculated by the phase-locked amplification algorithm.

5. The variable optical path laser gas detection method according to claim 4, characterized in that: The method for obtaining the minimum AD value of the electrical signal absorption area converted by the current photodetector is: Take the output signal wave of the current photoelectric detector within one detection cycle, set the total number of sampling points to M, and store the AD values ​​of the M sampling points in data1; The output signal wave is grouped into N sampling points; Suppose the AD value of the first sampling point in each group is a1, and the AD value of the N-1th sampling point is b1; the AD value of the second sampling point in each group is a2, and the AD value of the Nth sampling point is b2; Calculate sum1 = (a1 + b1) N / 2; Calculate sum2 = add the AD values ​​of the first sampling point in sequence to the sum of the AD values ​​of the N-1th sampling point; Subtract sum1 from sum2, take the absolute value, and store it in data; Calculate sum1'=(a1+b1)N / 2; Calculate sum2' = the sum of the AD values ​​starting from the second sampling point and added to the AD value of the Nth sampling point; Subtract sum1' from sum2', take the absolute value, and store it in data; Calculate the absolute value of sum1 and sum2 of each group, as well as the absolute value of sum1' and sum2', and store them in data respectively; Find the maximum value of data data and remember the position of the maximum value in data data; According to the position of the maximum value found in data data, the AD value of the corresponding position in data data1 is taken as the minimum AD value.

6. The variable optical path laser gas detection method according to claim 4, characterized in that: The method for calculating the gas detection result using the output signal of the current detection photoelectric detector is: The gas concentration, i.e. the gas detection result, can be output based on the absorption coefficient ε and the calibrated parameters.

7. A variable optical path harmonic laser gas detection system, comprising a multiple reflection optical cavity, a laser, a photodetector, and a processor connected to the laser via a drive circuit and connected to the photodetector via a signal processing circuit, characterized in that: There are multiple photoelectric detectors placed at different optical path positions of the detection light path of the multi-reflection optical cavity; When performing gas detection, the variable optical path laser gas detection method described in any one of claims 1 to 6 is adopted.

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