Variable path length laser gas detection method and system

Abstract

The application provides a variable optical path laser gas detection method and system, and the method is used for full-range gas detection of single gas component in a laser gas sensor. The specific method is as follows: a photodetector is arranged at each of different optical path positions of a detection light path of a multiple-reflection optical cavity; a preset gas concentration to be detected is small; a photodetector at a maximum optical path position is switched to a current detection photodetector; if an output signal of the current detection photodetector does not satisfy a switching threshold condition, the output signal of the current detection photodetector is used to calculate a gas detection result; otherwise, a photodetector at a next small optical path position is switched to the current detection photodetector for continuous judgment; the switching threshold condition is that a minimum AD value of an electric signal conversion absorption area of the current photodetector is 0, and a V 1f > V 2f > V 3f ; V 1f , V 2f , V 3f respectively are first, second and third harmonic amplitudes calculated by FFT.

Description

Technical Field

[0001] This invention relates to the field of laser gas detection technology, and more specifically, to a variable path laser gas detection method and system. Background Technology

[0002] A laser gas sensor is a sensor that uses laser spectroscopy to detect the concentration of a target gas in the environment. It mainly includes a laser, a multiple reflection optical cavity, a photodetector, and signal processing circuitry that processes and analyzes the electrical signal output from the photodetector.

[0003] The core challenges in designing single-component laser gas sensors include:

[0004] (1) Relationship between optical path length and detection limit

[0005] The detection limit is mainly determined by the sensor's sensitivity, which is directly proportional to the optical path length. According to the Beer-Lambert Law, the longer the optical path, the more fully the laser interacts with the target gas molecules, resulting in a stronger absorption signal and enabling the detection of lower concentrations of gas.

[0006] (2) Optical path length and saturation problem in high-concentration gas detection

[0007] When the gas concentration is high, the laser light will be excessively absorbed over a long optical path, leading to signal saturation or nonlinear response. This phenomenon limits the accuracy of the sensor in detecting high-concentration gases.

[0008] Therefore, in practical application design, if the designed optical path is too long, although the detection limit can be guaranteed, there will be a high concentration saturation problem; if the designed optical path is too short, although high concentrations of target gas can be detected, the detection limit cannot be guaranteed.

[0009] 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 currently the main challenge in the design of laser gas sensors. Summary of the Invention

[0010] Therefore, it is necessary to provide a laser-based method, system, and medium for detecting micro-leaks in multiple gases, addressing the aforementioned technical problems.

[0011] To achieve the above objectives, the first aspect of the present invention provides a variable-path laser gas detection method for full-range gas detection of a single gas component in a laser gas sensor, the method being as follows:

[0012] A photodetector is set at different optical path positions in the detection optical path of the multi-reflection optical cavity;

[0013] If the concentration of the gas to be detected is low, the photodetector at the maximum optical path position is switched to the current detection photodetector. If the output signal of the current detection photodetector does not meet the switching threshold condition, the output signal of the current detection photodetector is used to calculate the gas detection result; otherwise, the photodetector at the next short optical path position is switched to the current detection photodetector to continue the judgment.

[0014] The threshold switching condition is:

[0015] The minimum AD value of the absorption region of the current photodetector is 0, and it conforms to V. 1f >V 2f >V 3f ;

[0016] Among them, V 1f V 2f V 3f These are the first harmonic amplitude, second harmonic amplitude, and third harmonic amplitude calculated after performing FFT calculation on the electrical signal waveform converted by the current photodetector.

[0017] 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 as follows:

[0018] The method for obtaining the minimum AD value of the absorption region of the electrical signal converted by the current photodetector is as follows:

[0019] Take the output signal wave of the photodetector within one detection cycle. Let the total number of sampling points be M. The AD value of the M sampling points is stored in data1.

[0020] The output signal wave is grouped into groups of N sampling points.

[0021] 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; let the AD value of the second sampling point in each group be a2, and the AD value of the Nth sampling point be b2;

[0022] Calculate sum1 = (a1 + b1)N / 2;

[0023] Calculate sum2 = summing the AD values ​​from the first sampling point up to the (N-1)th sampling point;

[0024] Subtract sum1 from sum2, take the absolute value, and then store it in the data.

[0025] Calculate sum1' = (a1 + b1)N / 2;

[0026] Calculate sum2' = sum of the AD values ​​starting from the second sampling point and adding them sequentially up to the AD value of the Nth sampling point;

[0027] Subtract sum1' from sum2', take the absolute value, and then store it in the data.

[0028] Calculate the absolute value of subtracting sum1 from sum2 for each group, and the absolute value of subtracting sum1' from sum2', and then store them into the data data respectively;

[0029] Find the maximum value in the data and remember the position of the maximum value in the data.

[0030] The minimum AD value is obtained by taking the AD value at the corresponding position in data1 based on the position of the found maximum value in data1.

[0031] Based on the above, the method for calculating gas detection results using the output signal of the current photodetector is as follows:

[0032] The first harmonic amplitude V1f in the non-absorption region of the output signal waveform of the current photodetector is calculated using FFT, and then the second harmonic amplitude V2f in the absorption region or the entire output signal waveform is calculated using the lock-in amplification algorithm to obtain the absorption coefficient ε=V2f / V1f of the current photodetector.

[0033] The gas concentration, i.e., the gas detection result, can be output based on the absorption coefficient ε and the calibrated parameters.

[0034] A second aspect of this invention provides a variable-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:

[0035] A photodetector is set at different optical path positions in the detection optical path of the multi-reflection optical cavity;

[0036] If the concentration of the gas to be detected is high, the photodetector at the position with the shortest optical path is switched to the current detection photodetector. If the output signal of the current detection photodetector does not meet the switching threshold condition, the output signal of the current detection photodetector is used to calculate the gas detection result; otherwise, the photodetector at the next position with a long optical path is switched to the current detection photodetector to continue the judgment.

[0037] The threshold switching condition is:

[0038] 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;

[0039] Wherein, the absorption coefficient ε = V2f / V1f, V1f is the first 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 or the entire output signal waveform calculated by the lock-in amplification algorithm.

[0040] A third aspect of the present invention provides a variable optical path harmonic laser gas detection system, comprising a multi-reflection optical cavity, a laser, a photodetector, and a processor connected to the laser via a driving circuit and connected to the photodetector via a signal processing circuit. The photodetector comprises multiple photodetectors and is placed at different optical path positions in the detection optical path of the multi-reflection optical cavity.

[0041] When performing gas detection, the variable path laser gas detection method described above is used.

[0042] This invention has outstanding substantive features and significant progress compared to the prior art, specifically:

[0043] This invention enables full-range detection of a single-component target gas by placing a photodetector at different optical paths in a multi-reflection optical cavity and switching the photodetectors at different optical paths.

[0044] The method for obtaining the minimum AD value of the absorption region of the electrical signal converted by the current photodetector provided by the present invention can quickly and accurately find the minimum AD value so as to switch to a suitable current detection photodetector for gas concentration detection. Attached Figure Description

[0045] Figure 1 This is a diagram showing the placement of photodetectors at different optical paths according to the present invention.

[0046] Figure 2 This is a diagram of the single-component full-range gas detection method in Example 1.

[0047] Figure 3 This is a diagram of the single-component full-range gas detection method in Example 2.

[0048] Figure 4 This is a schematic diagram of the system structure in Example 3. Detailed Implementation

[0049] The technical solution of the present invention will be further described in detail below through specific embodiments.

[0050] The following describes some of the technical features involved in this invention:

[0051] AD value: refers to the AD value output by the signal processing circuit in the laser gas sensor.

[0052] Absorption region: This is the area where the laser wavelength matches the absorption line of the target gas; gas molecules absorb laser light of a specific wavelength. Non-absorption region: This is the area where the laser wavelength is not absorbed by the target gas. Typically, the signal to be analyzed obtained within one detection cycle T can be divided into the non-absorption region and the absorption region.

[0053] Example 1

[0054] like Figure 1 and Figure 2 As shown in the figure, this embodiment presents a variable optical path laser gas detection method for full-range gas detection of a single gas component in a laser gas sensor. Specifically, the method involves detecting PN at different optical path positions in the detection optical path of a multi-reflection optical cavity. i Each is equipped with a photodetector, i=1,2,…,n, where the smaller the value of i, the longer the optical path. In this embodiment, the concentration of the gas to be detected is preset to be low, so as to ensure the lower limit of detection first.

[0055] The specific testing method is as follows:

[0056] First, initialize the parameters;

[0057] Then a laser drive signal is generated, and the laser emits a detection laser to begin detection;

[0058] The processor processes the output electrical signal of the currently detecting photodetector, i.e., PN. i The output electrical signal of the photodetector at the location, i=1,2,…,n;

[0059] Get PN i The minimum AD value of the absorption region of the photodetector at the location where the electrical signal is converted. i At the same time, for PN i The electrical signal waveform converted by the photodetector at the location is used for FFT calculation to calculate the first harmonic amplitude V. 1f Second harmonic amplitude V 2f and the amplitude of the third harmonic V 3f ;

[0060] If PN i The minimum AD value of the absorption region of the photodetector at the location where the electrical signal is converted. i Not equal to 0, and does not conform to V 1f >V 2f >V 3f Then use PN i The output signal of the photodetector at the current location is used to calculate the gas detection result; otherwise, let i = i + 1, and switch the current detection photodetector to PN. i The photodetector at the location continues to make a judgment;

[0061] Switch sequentially until a suitable photodetector outputs gas detection results.

[0062] 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:

[0063] Take the output signal wave of the photodetector within one detection cycle. Let the total number of sampling points be M. The AD value of the M sampling points is stored in data1.

[0064] The output signal wave is grouped into groups of N sampling points.

[0065] 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; let the AD value of the second sampling point in each group be a2, and the AD value of the Nth sampling point be b2;

[0066] Calculate sum1 = (a1 + b1)N / 2;

[0067] Calculate sum2 = summing the AD values ​​from the first sampling point up to the (N-1)th sampling point;

[0068] Subtract sum1 from sum2, take the absolute value, and then store it in the data.

[0069] Calculate sum1' = (a1 + b1)N / 2;

[0070] Calculate sum2' = sum of the AD values ​​starting from the second sampling point and adding them sequentially up to the AD value of the Nth sampling point;

[0071] Subtract sum1' from sum2', take the absolute value, and then store it in the data.

[0072] Calculate the absolute value of subtracting sum1 from sum2 for each group, and the absolute value of subtracting sum1' from sum2', and then store them into the data data respectively;

[0073] Find the maximum value in the data and remember the position of the maximum value in the data.

[0074] The minimum AD value is obtained by taking the AD value at the corresponding position in data1 based on the position of the found maximum value in data1.

[0075] In some exemplary embodiments, the method for calculating the gas detection result using the output signal of the currently detected photodetector is as follows:

[0076] The first harmonic amplitude V1f in the non-absorption region of the output signal waveform of the current photodetector is calculated using FFT, and then the second harmonic amplitude V2f in the absorption region or the entire output signal waveform is calculated using the lock-in amplification algorithm to obtain the absorption coefficient ε=V2f / V1f of the current photodetector.

[0077] The gas concentration, i.e., the gas detection result, can be output based on the absorption coefficient ε and the calibrated parameters.

[0078] It should be noted that:

[0079] According to Beer-Lambert's law, the absorption of light is directly proportional to the gas concentration and the optical path length:

[0080] A=ε·c· l

[0081] Where: A is absorbance, ε is absorption coefficient (unit: L·mol⁻¹·cm⁻¹), and c is gas concentration (unit: mol / L). l Optical path length (unit: cm);

[0082] Then the gas concentration c = A / (ε· l );

[0083] By pre-calibrating the parameters absorbance A and optical path length l Then, the output gas concentration c, i.e., the gas detection result, can be obtained based on the absorption coefficient ε.

[0084] It should be noted that in this embodiment, after each gas detection result is obtained and the detection is completed, the system switches back to the photodetector at the default maximum optical path position.

[0085] Example 2

[0086] like Figure 3 As shown in the figure, this embodiment presents a variable optical path laser gas detection method for full-range gas detection of a single gas component in a laser gas sensor. Specifically, the method involves detecting PN at different optical path positions in the detection optical path of a multi-reflection optical cavity. i Each is equipped with a photodetector, i=1,2,…,n, where the smaller the value of i, the longer the optical path. In this embodiment, the concentration of the gas to be detected is preset to be high, ensuring that the gas is unsaturated at high concentration.

[0087] The specific testing method is as follows:

[0088] First, initialize the parameters;

[0089] Then a laser drive signal is generated, and the laser emits a detection laser to begin detection;

[0090] The processor processes the output electrical signal of the currently detecting photodetector, i.e., PN.i The output electrical signal of the photodetector at the location, i=n,n-1,…2,1;

[0091] Get PN i The minimum AD value of the absorption region of the photodetector at the location where the electrical signal is converted. i At the same time, for PN i The electrical signal waveform converted by the photodetector at the location is used to calculate the first harmonic amplitude V1f in the non-absorption region using FFT. Then, the second harmonic amplitude V2f in the absorption region or the entire output signal waveform is calculated using a lock-in amplification algorithm to obtain the PN junction. i The absorption coefficient ε of the photodetector at the location i =V2f / V1f

[0092] If PN i The minimum AD value of the absorption region of the photodetector at the location where the electrical signal is converted. i Greater than 0, and ε i =0, then use PN i The output signal of the photodetector at the location is used to calculate the gas detection result; otherwise, let i = i-1, and switch the current detection photodetector to PN. i The photodetector at the location continues to make a judgment;

[0093] Switch sequentially until a suitable photodetector outputs gas detection results.

[0094] 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:

[0095] Take the output signal wave of the photodetector within one detection cycle. Let the total number of sampling points be M. The AD value of the M sampling points is stored in data1.

[0096] The output signal wave is grouped into groups of N sampling points.

[0097] 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; let the AD value of the second sampling point in each group be a2, and the AD value of the Nth sampling point be b2;

[0098] Calculate sum1 = (a1 + b1)N / 2;

[0099] Calculate sum2 = summing the AD values ​​from the first sampling point up to the (N-1)th sampling point;

[0100] Subtract sum1 from sum2, take the absolute value, and then store it in the data.

[0101] Calculate sum1' = (a1 + b1)N / 2;

[0102] Calculate sum2' = sum of the AD values ​​starting from the second sampling point and adding them sequentially up to the AD value of the Nth sampling point;

[0103] Subtract sum1' from sum2', take the absolute value, and then store it in the data.

[0104] Calculate the absolute value of subtracting sum1 from sum2 for each group, and the absolute value of subtracting sum1' from sum2', and then store them into the data data respectively;

[0105] Find the maximum value in the data and remember the position of the maximum value in the data.

[0106] The minimum AD value is obtained by taking the AD value at the corresponding position in data1 based on the position of the found maximum value in data1.

[0107] In some exemplary embodiments, the method for calculating the gas detection result using the output signal of the currently detected photodetector is as follows:

[0108] The gas concentration, i.e., the gas detection result, can be output based on the absorption coefficient ε and the calibrated parameters.

[0109] It should be noted that:

[0110] According to Beer-Lambert's law, the absorption of light is directly proportional to the gas concentration and the optical path length:

[0111] A=ε·c· l

[0112] Where: A is absorbance, ε is absorption coefficient (unit: L·mol⁻¹·cm⁻¹), and c is gas concentration (unit: mol / L). l Optical path length (unit: cm);

[0113] Then the gas concentration c = A / (ε· l );

[0114] By pre-calibrating the parameters absorbance A and optical path length l Then, the output gas concentration c, i.e., the gas detection result, can be obtained based on the absorption coefficient ε.

[0115] It should be noted that in this embodiment, after each gas detection result is obtained and the detection is completed, the system switches back to the photodetector at the default minimum optical path position.

[0116] Example 3

[0117] 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 via a driving circuit and connected to the photodetector via a signal processing circuit; multiple photodetectors are placed at different optical path positions in the detection optical path of the multi-reflection optical cavity, such as... Figure 4 As shown;

[0118] When performing gas detection, the variable path laser gas detection method described in Example 1 or Example 2 can be used.

[0119] 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 preferred embodiments, those skilled in the art should understand that modifications can still be made to the specific implementation of the present invention or equivalent substitutions can be made to some technical features without departing from the spirit of the technical solutions of the present invention, and all such modifications and substitutions should be covered within the scope of the technical solutions claimed in the present invention.

Claims

1. A variable path length laser gas detection method, characterized by, The method for single gas component full-range gas detection in a laser gas sensor is: A photoelectric detector is arranged at each different optical path position of a detection light path of a multi-reflection optical cavity; A preset small gas concentration is detected, a photoelectric detector at a maximum optical path position is switched to a current detection photoelectric detector, if an output signal of the current detection photoelectric detector does not satisfy a switching threshold condition, a gas detection result is calculated by using the output signal of the current detection photoelectric detector; otherwise, a photoelectric detector at a next small optical path position is switched to the current detection photoelectric detector for continuous judgment; The switching threshold condition is: The minimum AD value of the current photoelectric detector converted electrical signal absorption region is 0, and meets V 1f > V 2f > V 3f ; wherein V 1f , V 2f , V 3f are the first, second and third harmonic amplitudes, respectively, calculated after performing an FFT calculation on the electrical signal waveform converted by the current photodetector. The method for obtaining a minimum AD value of an electric signal absorption area converted by a current photoelectric detector is: An output signal wave of the current photoelectric detector in a detection period is taken, a total sampling point number is M, and AD values of the M sampling points are stored in data data1; The output signal wave is grouped in N sampling points as a group; An AD value of a first sampling point in each group is a1, and an AD value of an N-1 sampling point is b1; an AD value of a second sampling point in each group is a2, and an AD value of an N sampling point is b2; sum1=(a1+b1)N / 2 is calculated; sum2 is calculated by sequentially adding AD values from the first sampling point to the AD value of the N-1 sampling point; An absolute value of a subtraction result of sum1 and sum2 is stored in data data; sum1'=(a1+b1)N / 2 is calculated; sum2' is calculated by sequentially adding AD values from the second sampling point to the AD value of the N sampling point; An absolute value of a subtraction result of sum1' and sum2' is stored in data data; Absolute values of the subtraction results of sum1 and sum2 of each group and the subtraction results of sum1' and sum2' are respectively calculated and stored in data data; A maximum value is searched in data data, and a position of the maximum value in data data is remembered; An AD value of a corresponding position in data data1 is obtained according to the position of the maximum value in data data, and the AD value is the minimum AD value.

2. The variable optical length laser gas detection method of claim 1, wherein, The method for calculating a gas detection result by using an output signal of a current detection photoelectric detector is: A first harmonic amplitude V1f of a non-absorption area of an output signal waveform of the current detection photoelectric detector is calculated by using FFT, a second harmonic amplitude V2f of an absorption area or an entire output signal waveform is calculated by using a phase-locked amplification algorithm, and an absorption coefficient ε=V2f / V1f of the current detection photoelectric detector is obtained; According to the absorption coefficient ε and calibrated parameters, a gas concentration, that is, a gas detection result, can be output.

3. A variable path length laser gas detection method, characterized by, The method for single gas component full-range gas detection in a laser gas sensor is: A photoelectric detector is arranged at each different optical path position of a detection light path of a multi-reflection optical cavity; The preset gas concentration to be detected is large, the photoelectric detector at the minimum optical path position is switched to a current detection photoelectric detector first, if the output signal of the current detection photoelectric detector does not satisfy a switching threshold condition, the output signal of the current detection photoelectric detector is used to calculate a gas detection result; otherwise, the photoelectric detector at the next large optical path position is switched to the current detection photoelectric detector to continue to judge; The switching threshold condition is that: The minimum AD value of the absorption area of the converted electrical signal of the current photoelectric detector is greater than 0, and the absorption coefficient ε = 0; 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 photoelectric detector calculated by using the FFT, and V2f is the second harmonic amplitude of the absorption area or the entire output signal waveform calculated by using the phase-locked amplification algorithm; The method for obtaining the minimum AD value of the absorption area of the converted electrical signal of the current photoelectric detector is that: The output signal wave of the current detection photoelectric detector in one detection period is taken, the total sampling point number is M, and the AD values of the M sampling points are stored in data data1; The output signal wave is grouped in N sampling points as a group; 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; sum1 = (a1+b1)N / 2 is calculated; sum2 = the sum of the AD values from the first sampling point to the AD value of the N-1th sampling point is calculated; The absolute value of the difference between sum1 and sum2 is taken and stored in data data again; sum1' = (a1+b1)N / 2 is calculated; sum2' = the sum of the AD values from the second sampling point to the AD value of the Nth sampling point is calculated; The absolute value of the difference between sum1' and sum2' is taken and stored in data data again; The absolute values of the differences between sum1 and sum2 and the absolute values of the differences between sum1' and sum2' of each group are calculated and stored in data data respectively; The maximum value of data data is found, and the position of the maximum value in data data is remembered; The AD value of the corresponding position in data data1 is taken according to the position of the found maximum value in data data, which is the minimum AD value.

4. The variable optical length laser gas detection method of claim 3, wherein, The method for calculating the gas detection result using the output signal of the current detection photoelectric detector is that: According to the absorption coefficient ε and the calibrated parameters, the gas concentration, i.e. the gas detection result, can be output.

5. A variable optical path harmonic laser gas detection system, comprising a multiple reflection optical cavity, a laser, a photoelectric detector, and a processor connected with the laser through a driving circuit and connected with the photoelectric detector through a signal processing circuit, characterized in that: The photoelectric detector is multiple and placed at different optical path positions of the detection light path of the multiple reflection optical cavity; When gas detection is performed, the variable optical path laser gas detection method of any one of claims 1-4 is used.

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