Light interference gas sensor structure and self-calibration method

Abstract

The invention belongs to the technical field of gas concentration measurement, and particularly provides a light interference gas sensor structure and a self-calibration method. The sensor is composed of a measuring gas chamber, a photosensitive element and the like, and the measuring gas chamber is of a four-cavity structure. According to the working principle, light emitted by the same light source passes through different gas chambers, optical path difference and phase shift are generated due to different gas concentrations, then interference fringes are formed, and gas concentration detection is achieved by detecting the movement amount of the fringes. Zero-point self-calibration is achieved through comparison of two interference fringes generated by the four gas chambers, the non-linear compensation device has the advantages of being high in non-linear compensation precision, capable of avoiding reading errors caused by mechanical vibration and temperature changes and the like, and a reliable scheme is provided for accurate measurement of gas concentration.

Description

Technical Field

[0001] The present invention belongs to the technical field of gas concentration measurement, and specifically relates to a gas detection method, system and device based on the optical interference optical principle. Background Art

[0002] Gas concentration monitoring plays a crucial role in safety production. By timely and accurately monitoring the gas components and concentrations in the industrial environment, enterprises can effectively prevent potential safety risks. During the industrial production process, certain gases are harmful to the human body and may even cause explosions or fires. Gas concentration monitoring equipment can monitor the concentrations of these gases in real time. Once the safety threshold is exceeded, the system will issue an alarm to remind employees to take timely measures to prevent accidents, improve the production safety level, help protect the personal safety of employees, and also help protect production equipment and the environment. In addition, gas concentration data can also provide strong support for production decision-making and process optimization, thereby improving product quality and production efficiency.

[0003] The measurement accuracy of gas concentration sensors is crucial for safety monitoring. However, in practical applications, due to factors such as sensor contamination, aging or physical damage, the accuracy and stability of zero calibration are often affected. In addition, environmental factors such as high temperature, high humidity, and dust may also interfere with the sensor measurement results. These problems may cause errors in the sensor after zero calibration, thus affecting its accuracy and reliability in practical applications. Currently, the zero calibration method of gas sensors mainly uses standard gas samples for calibration through manual methods. The operation process is cumbersome, the calibration work has a large labor intensity, and it requires a lot of manpower and material resources, making it difficult to meet the requirements of real-time and accurate measurement. For optical interference gas sensors, the existing technologies still have deficiencies in suppressing non-linear errors and suppressing zero drift, making it difficult to achieve real-time and accurate measurement of gas concentration. It is necessary to adopt a new design from aspects such as the optical path structure, signal acquisition, and calibration method of the sensor, make full use of the advantages of the relative measurement of the optical interference optical path difference, realize the automatic calibration of the sensor during actual use, suppress zero drift, and improve the measurement accuracy and stability. Summary of the Invention

[0004] The present invention relates to an optical interference gas sensor structure, which includes a measurement gas chamber, a photosensitive element, a linear light source, a plane mirror, a triangular prism, and a compensation prism; the measurement gas chamber adopts a four-chamber structure, including three reference gas chambers and one sampling gas chamber; two rectangular reference gas chambers in the upper layer are filled with reference gas, one rectangular reference gas chamber in the lower layer is filled with reference gas, and the other rectangular sampling gas chamber in the lower layer is filled with the gas to be measured; the two rectangular reference gas chambers in the upper layer, the rectangular reference gas chamber in the lower layer, and the rectangular sampling gas chamber in the lower layer are arranged side by side vertically; the two compensation prisms are located between the upper gas chamber and the plane mirror; the photosensitive element uses a linear CCD to collect the interference fringe signals in the vertical direction.

[0005] Further, the light emitted by the linear light source is reflected by the plane mirror coated with a total reflection film, and is divided into two parallel beams of light, which respectively pass through the rectangular reference gas chambers in the upper and lower layers to form two sets of light. After passing through the rectangular reference gas chambers, they are horizontally incident on the triangular prism, and after being reflected twice by the triangular prism, they respectively pass through another rectangular reference gas chamber in the upper layer and the rectangular sampling gas chamber in the lower layer, and are incident on the plane mirror coated with a total reflection film on the back for reflection. Two sets of upper and lower light rays respectively generate interference fringes at the photosensitive element. The interference fringes are divided into upper and lower parts, denoted as the first interference fringe and the second interference fringe.

[0006] Further, the compensation prism is provided with a mechanical adjustment device, which can change the optical path difference of the light to achieve mechanical zero adjustment and ensure that the interference fringes always exist.

[0007] Further, the self-calibration method of the optical interference gas sensor structure is characterized by including the following steps:

[0008] 1. The photosensitive element respectively detects the first interference fringe and the second interference fringe;

[0009] 2. The distribution period and average phase of the first interference fringe and the second interference fringe are respectively obtained through signal analysis methods;

[0010] 3. Taking the average phase of the first fringe as the zero-point reference, the average phase of the second fringe is calibrated for zero-point offset;

[0011] 4. According to the corresponding relationship between the fringe phase and the gas concentration, the concentration value of the measured gas is further obtained.

[0012] Further, the linear light source is a visible light source with a single wavelength, and both the linear light source and the photosensitive element are vertically installed. The light emitted by the linear light source generates interference fringes at the photosensitive element after passing through the optical path system.

[0013] Further, the front and back surfaces of the measurement gas chamber are sealed and isolated from the outside by high-transparency planar prisms, and the other directions are sealed and isolated from the outside by metal. The gas chambers are sealed and isolated from each other by metal. There is a communication area between the three reference gas chambers to ensure that the gases in the three reference gas chambers are exactly the same.

[0014] Further, the peak detection algorithm is used to detect the gray peak of the received optical interference fringe, obtain the effective position for positioning the optical interference fringe, and compare the effective positions of the processed optical interference fringes generated by the upper and lower groups of gas chambers to realize the measurement of the gas concentration.

[0015] Further, for the self-calibration method, the optical interference signal is preprocessed, specifically including:

[0016] 1. Perform gray-scale processing on the signal. Use gamma transformation to map the wider gray-scale range in the input image to the narrower gray-scale range in the output image, compress the dark pixel values, and expand the high gray-level values.

[0017] 2. Remove the noise from the collected image. Use median filtering to replace the value of each pixel with the median of the surrounding pixel values to remove salt-and-pepper noise.

[0018] 3. Extract the optical interference signal after image processing, obtain the peak position of each interference fringe, and use it as the comparison position of the optical interference fringes in the upper and lower gas chambers.

[0019] Further, for the self-calibration method, the gas concentration detection process is as follows:

[0020] 1. Fill the reference gas chamber with the standard reference gas from the ventilation port.

[0021] 2. Open the intake port of the sampling gas chamber and place it in the environment to be measured. After the sampling gas chamber is filled with the gas to be measured, the linear light source emits light, which is incident on the plane mirror with a total reflection film on the back and undergoes both reflection and refraction, and finally two parallel light rays are emitted from the plane mirror.

[0022] 3. The two parallel light rays pass through the gas chamber structure and then enter the triangular prism, where they undergo optical path reflection and re-enter the gas chamber structure. The light rays emitted by the line light source are split by the triangular prism and pass through the upper standard gas chamber and the lower sampling gas chamber respectively, and then enter the plane mirror through the compensation prism. After being reflected by the front and back surfaces of the plane mirror with a total reflection film, interference fringes are generated respectively and enter the photosensitive element.

[0023] 4. Combining the reference interference fringes in the upper half of the photosensitive element, the accurate concentration of the gas in the sampling gas chamber can be accurately obtained by measuring the light intensity in the lower half of the photosensitive element.

[0024] Furthermore, for the self-calibration method described above, the program algorithm logic for automatically tracking and detecting the fringe shift amount is as follows:

[0025] 1. Perform gray-scale transformation on the original signal of the collected fringes, adjust the gray-scale range of the signal, and filter the signal to remove the noise mixed in the process of collecting the signal;

[0026] 2. For the signal after digital image processing, convert the signal from the time domain to the frequency domain through fast Fourier transform, and calculate the average distribution period and the average phase ψ;

[0027] 3. Based on the fringe period and the average phase, divide the fringe signal within the window into n intervals;

[0028] 4. Perform extreme value search in each interval to obtain the extreme values of each interval;

[0029] 5. Compare the extreme values of adjacent intervals, judge whether they are valid extreme values by comparing the distances of the horizontal axis coordinates, and obtain a new set of valid extreme values;

[0030] 6. Compare the abscissa of the valid extreme values with the position of the previous fringe extreme value coordinates, and select the abscissas of the two extreme values with the closest Euclidean distance;

[0031] 7. According to the previous 5 values of the fringe extreme value coordinates, use polynomial fitting curve, compare the change trends, and select the value with the smallest curve interpolation as the effective fringe position for measurement. The present invention has the following beneficial effects:

[0032] (1) The zero self-calibration of the sensor can be realized by comparing the two interference fringes generated by the four air chambers;

[0033] (2) The non-linear compensation accuracy is higher, improving the product performance;

[0034] (3) The sensor reading error caused by mechanical vibration and temperature change is avoided;

[0035] (4) In the part of the received optical interference signal, an image processing algorithm is adopted, and gamma transformation and median filtering are used to improve the accuracy of positioning the optical interference signal;

[0036] (5) For the case where the fringe shift amount exceeds one bright and dark period, targeted design is carried out in the software program to realize the automatic tracking and monitoring of the fringe shift amount. Description of the Drawings

[0037] Figure 1 is a schematic structural diagram of the optical interference gas concentration sensor system in the present invention;

[0038] Figure 2It is a three-dimensional structure diagram of the air chamber in the present invention;

[0039] Figure 3 It is an explanation of the uses of different air chambers in the present invention;

[0040] Figure 4 It is a flowchart for detecting the gas concentration in the present invention;

[0041] Figure 5(a) is a schematic diagram of the interference fringes generated by the upper and lower layers of air chambers in the present invention;

[0042] Figure 5(b) is a schematic diagram of the interference fringes during concentration detection in the present invention;

[0043] Figure 6 It is a schematic diagram of the interference fringes before and after image processing in the present invention.

[0044] Among them, there are a linear light source (1), a plane mirror (2), a measurement air chamber (3), a triangular prism (4), a compensation prism (5), and a photosensitive element (6). Specific embodiments

[0045] The following further describes the embodiments of the present invention in detail in conjunction with the drawings and embodiments. The following embodiments are used to illustrate the present invention, but cannot be used to limit the scope of the present invention.

[0046] The sensor system proposed by the present invention is used for gas concentration measurement, with high measurement accuracy and good stability. A beam of light emitted by a linear light source of a single wavelength is reflected and refracted by a plane mirror and then divided into two beams of light. After passing through a standard reference air chamber filled with air and a sampling air chamber filled with the gas to be measured respectively, when the two beams of light meet again after being reflected and refracted by the plane mirror, interference fringes will be generated. With the change of the concentration of the gas to be measured, the position of the interference fringes will change accordingly;

[0047] In the optical path design, the linear light source is a visible light source of a single wavelength, the photosensitive element uses a linear CCD device, the linear light source and the photosensitive element are both vertically installed, and the light emitted by the linear light source generates interference fringes at the photosensitive element after passing through the optical path system; the measurement air chamber adopts a four-chamber structure with upper and lower layers. The two reference air chambers in the upper layer are located in the upper half of the air chamber structure and are arranged horizontally side by side. One reference air chamber and one sampling air chamber in the lower layer are located in the lower half of the air chamber structure and are arranged horizontally side by side. Among them, the left side is the reference air chamber and the right side is the sampling air chamber. The front and back sides of the air chamber are sealed and isolated from the outside with high-transmission plane prisms, and the other directions are sealed and isolated from the outside with metal;

[0048] The metal seals are used to isolate between the gas chambers. There is a communication area among the three reference gas chambers to ensure that the gases in the three reference gas chambers are completely the same. The upper two reference gas chambers are used to generate standard interference fringes as the benchmark at 0 concentration. The lower reference gas chamber and the sampling gas chamber are used to generate interference fringes at the current gas concentration for methane concentration measurement.

[0049] The plane mirror is located on the left side of the gas chamber structure. Total reflection films are coated on the backs of the two reflecting surfaces for reflecting the light emitted by the line light source. The prism is located on the right side of the gas chamber structure for reflecting the light passing through the gas chamber. After the light is reflected twice by the prism, it is exactly parallel to the incident light, so that the reflected light can be incident into the gas chamber again.

[0050] After the light emitted by the line light source reflected by the prism is respectively incident into the upper reference gas chamber and the lower sampling gas chamber, it passes through the highly transparent plano-convex lenses on both sides of the gas chamber and is incident on the plane mirror on the left side. After being reflected by the front and back surfaces of the plane mirror, the light converges into a beam and generates interference fringes on the photosensitive elements respectively. By comparing the two interference fringes, the measurement of the gas concentration is realized.

[0051] In the process of comparing the two interference fringes, the peak detection algorithm is used to detect the gray peak of the received optical interference fringes to obtain the effective position for positioning the optical interference fringes, and the effective positions of the optical interference fringes generated by the upper and lower groups of gas chambers after processing are compared to realize the measurement of the gas concentration.

[0052] The present invention will be further described below in conjunction with the drawings and examples:

[0053] As Figure 1As shown in the figure, the present invention provides a high-precision optical interference methane concentration sensing system. The sensing system includes a line light source 1, a plane mirror 2 coated with a total reflection film, a chamber structure 3 composed of four chambers, a triangular prism 4, a compensation prism 5, and a photosensitive element 6. The plane mirror 2 coated with a total reflection film is arranged on the left side of the chamber structure 3. The horizontal angle between the plane mirror 2 and the chamber 3 is 45°, so that the light incident on the plane mirror can be horizontally emitted after passing through the plane mirror 2 coated with a total reflection film. The line light source 1 is arranged below the plane mirror 2, also at an angle of 45°, so that the light emitted by the line light source 1 can be parallelly incident into the chamber structure after being refracted by the plane mirror 2. The photosensitive element 6 is also arranged below the plane mirror 2. The light returning from the chamber can be reflected by the front and back surfaces of the plane mirror 2 and converged to be incident on the photosensitive element 6. The line light source 1 is vertically aligned, so that the emitted light can pass through the upper reference chamber and the lower sampling chamber respectively. The photosensitive element 6 is also vertically aligned. The two beams of light emitted from the chamber 3 and reflected and converged by the front and back surfaces of the plane mirror 2 can be incident into the photosensitive element 6 and generate interference fringes. The triangular prism 4 is installed on the other side of the chamber 3 and is used to reflect the light emitted by the line light source 1 after passing through the chamber 3. Both back surfaces of the two reflecting surfaces of the triangular prism 4 are coated with a total reflection film for reflecting light. The light emitted from the line light source 1 is reflected by the front and back surfaces of the plane mirror 2 and is divided into two beams of light, which respectively pass through the upper left reference chamber, the upper right reference chamber, the lower left sampling chamber, and the lower right reference chamber to form four beams of light. The four beams of light are incident on the triangular prism 4 and are reflected by its two surfaces and then return to the upper right reference chamber and the lower right reference chamber. Before the two beams of reflected light in the upper reference chamber are incident on the plane mirror 2, they are incident into the compensation prism 5. The purpose of the compensation prism 5 is to generate initial interference fringes when the methane concentration is 0, and a mechanical rotation structure is provided at the position of the compensation prism 5, which can be used to adjust the angle of the compensation prism 5 to achieve mechanical zero adjustment of the sensor. The two beams of light passing through the compensation prism 5 are incident into the plane mirror 2, and after being reflected by the two surfaces coated with a total reflection film, they are converged into a beam of light and incident into the upper half region of the photosensitive element 6; the two beams of light in the lower reference chamber are incident into the compensation prism 5 before being incident on the plane mirror 2, pass through the compensation prism and are incident into the plane mirror 2, and after being reflected by the two surfaces coated with a total reflection film, they are converged into a beam of light and incident into the lower half region of the photosensitive element 6.

[0054] This type of optical interference gas sensor can measure gases in different concentration ranges. By establishing an index table of the basic numerical range of the concentration of the gas to be measured and the change in the light intensity of the interference fringes on the photoelectric detection element according to the change in the light intensity of the interference fringes;

[0055] At the same time, according to the calculation of the optical path difference in this optical path, the relationship between the optical path difference and the gas concentration can be obtained. Let the gas concentration be x, the chamber length be L, and the refractive index of the compensation prism be n c, if the thickness of the compensating prism is h and the tilting angles of the two compensating prisms are α and β respectively, the relationship between the optical path difference and the gas concentration is as follows:

[0056]

[0057] On this basis, when accurately calculating the gas concentration, the linear interpolation method is used to accurately calculate the gas concentration. By comparing the light intensity difference between the upper and lower optical paths of the photosensitive element, the concentration of the gas to be measured can be accurately obtained by measuring the light intensity of the interference fringes.

[0058] In the process of comparing the positions of the interference fringes, the optical interference signal is preprocessed first:

[0059] Step 1: Perform grayscale processing on the signal. Use gamma transformation to map the relatively wide grayscale range in the input image to a relatively narrow grayscale range in the output image, compress the dark pixel values, and expand the high grayscale values at the same time. The relationship is as follows: s = cr 2.7

[0060] Step 2: Remove the noise from the collected image. Use median filtering to replace the value of each pixel with the median of the pixel values around it to remove salt-and-pepper noise.

[0061] Step 3: Extract the optical interference signal after image processing to obtain the peak position of each interference fringe, which is used as the comparison position of the optical interference fringes in the upper and lower gas chambers.

[0062] The process of gas concentration detection is as Figure 4 shown, and the specific process is as follows:

[0063] Step 1: Fill the reference gas chamber with the standard reference gas from the gas inlet.

[0064] Step 2: Open the gas inlet of the sampling gas chamber and place it in the environment to be measured. After the sampling gas chamber is filled with the gas to be measured, the linear light source 1 emits light, which is incident on the plane mirror with a total reflection film on the back and undergoes both reflection and refraction, and finally exits the plane mirror as two parallel light rays.

[0065] Step 3: The two parallel light rays described in Step 2, after passing through the gas chamber structure, are incident on the triangular prism and undergo optical path reflection, and then re-enter the gas chamber structure. The light rays emitted by the line light source are split by the triangular prism and pass through the upper standard gas chamber and the lower sampling gas chamber respectively, and then enter the plane mirror through the compensating prism. After being reflected by the front and back surfaces of the plane mirror with a total reflection film, interference fringes are generated respectively and are incident on the photosensitive element.

[0066] Step 4: Combining with the reference interference fringes in the upper half of the photosensitive element, the accurate concentration of the gas in the sampling gas chamber can be accurately obtained through the measured light intensity in the lower half of the photosensitive element.

[0067] The detection principle is as follows:

[0068] When light rays emit from a linear light source and pass through a plane mirror with a total reflection film on the back, refraction and reflection occur, forming two light rays. These two light rays pass through the gas chamber and, after reflection by a triangular prism, pass through the gas chamber again, then through a compensation prism, and after reflection and refraction by a plane mirror with a total reflection film on the back, an optical path difference will be generated, thus forming interference fringes on the photosensitive element. When the concentration of the gas to be measured in the lower sampling gas chamber changes, the interference fringes formed in the lower half of the photosensitive element will shift. According to the amount of shift of the interference fringes and by comparing with the initial position of the interference fringes formed in the upper half of the photosensitive element, the refractive index of the gas in the lower sampling gas chamber can be deduced, and then based on the relationship between the gas refractive index and the concentration of the gas, the concentration of the sampled gas can be obtained. This is the basic principle of using the optical interference method to measure gas concentration.

[0069] The program logic for measurement is as follows:

[0070] First, the photosensitive element respectively collects the original signals of the interference fringes generated when the light source passes through the upper and lower gas chambers, and calculates the average distribution period and average phase of the upper layer fringe signal;

[0071] Based on the average distribution period and average phase of the interference fringes generated in the upper gas chamber, the fringe signals within the window are divided into n intervals;

[0072] Extreme value searches are carried out in each interval to obtain the extreme values inside each interval as the effective positions of the fringes. Since the optical path differences are different when the light source passes through the upper and lower gas chambers, there will be a significant phase difference between the interference fringes in the upper and lower parts. The effective positions of the fringes obtained from the interval extreme values of the interference fringes in the upper and lower parts can be used to obtain the amount of shift of the interference fringes;

[0073] Continuous fringe original data are selected in the neighborhood of the positions of the upper and lower interference fringes. The fringe original data are grayscale processed, the noise in the fringe original signal is removed using median filtering, and the average value of the maximum positions of the fringes is obtained according to the centroid method as the fringe position for value taking;

[0074] The value-taking fringe coordinate axes of the upper and lower interference fringes are compared, combined with the calibration data, and non-linear compensation is carried out to finally obtain the output value of the gas concentration.

[0075] The program algorithm logic for automatic tracking detection of the fringe shift amount is as follows:

[0076] (1) As described above, perform grayscale transformation on the original signal of the collected fringes, adjust the grayscale range of the signal, and filter the signal to remove the noise mixed in the process of collecting the signal;

[0077] (2) For the signal after digital image processing, the signal is transformed from the time domain to the frequency domain through fast Fourier transform, and the average distribution period T and average phase ψ of the fringe signal are calculated;

[0078] (3) Based on the fringe period and average phase, the fringe signal within the window is divided into n intervals, namely [x 0 , x 1 , [x 1 , x 2 , …, [x n-1 , x n ;

[0079] (4) Extreme value search is performed in each interval to obtain the extreme values x M1 , x M2 , …, x Mn of each interval;

[0080] (5) Compare the extreme values of adjacent intervals, and judge whether they are valid extreme values by comparing the distances of the horizontal axis coordinates, and obtain a new set of valid extreme values;

[0081] (6) Since the amount of movement of the fringe does not change suddenly, compare the abscissa of the valid extreme value with the position of the previous fringe extreme value coordinate , and select the abscissas x A , x B of the two extreme values with the closest Euclidean distance;

[0082] (7) According to the first 5 values of the fringe extreme value coordinates , use polynomial fitting curve, and compare the change trends with x A , x B , and select the value with the smallest curve interpolation as the effective fringe position for measurement.

[0083] The specific embodiments described above further elaborate on the purpose, technical solutions, and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not used to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. An optical interference gas sensor structure, characterized in that: The invention comprises a measuring gas chamber (3), a photosensitive element (6), a line light source (1), a plane mirror (2), a prism (4), and a compensation prism (5); the measuring gas chamber (3) adopts a four-chamber structure, comprising three reference gas chambers and one sampling gas chamber; two rectangular reference gas chambers on the upper layer are filled with reference gas, one rectangular reference gas chamber on the lower layer is filled with reference gas, and another rectangular sampling gas chamber on the lower layer is filled with gas to be measured; the two rectangular reference gas chambers on the upper layer are arranged side by side in a vertical row with the rectangular reference gas chamber on the lower layer and the rectangular sampling gas chamber on the lower layer; the two compensation prisms (5) are located between the upper gas chamber and the plane mirror (2); the photosensitive element (6) adopts a linear CCD to collect interference fringe signals in the vertical direction.

2. The optical interference gas sensor structure according to claim 1, characterized in that: The light emitted by the line light source (1) is reflected by the plane mirror (2) coated with a total reflection film, and then divided into two parallel beams of light, and respectively pass through the upper and lower rectangular reference air chambers to form upper and lower groups of light. After passing through the rectangular reference air chamber, the light is horizontally incident on the prism (4), and after being reflected twice by the prism (4), the light is respectively passed through another upper rectangular reference air chamber and a lower rectangular sampling air chamber, and then incident on the plane mirror (2) coated with a total reflection film on the back side to reflect. The upper and lower groups of light respectively generate interference fringes at the photosensitive element (6), and the interference fringes are divided into upper and lower parts, which are recorded as first interference fringes and second interference fringes.

3. The optical interference gas sensor structure according to claim 1, characterized in that: The compensation prism (5) is provided with a mechanical adjustment device, which can change the optical path difference of the light to achieve mechanical zero adjustment and ensure that the interference fringes always exist.

4. A self-calibration method based on the optical interference gas sensor structure according to any one of claims 1 to 3, characterized in that: The following steps are involved:

1. The photosensitive element (6) detects the first interference fringe and the second interference fringe respectively; 2. Obtaining the distribution period and average phase of the first interference fringe and the second interference fringe respectively by using a signal analysis method; 3. Using the average phase of the first fringe as the zero point reference, the average phase of the second fringe is calibrated for zero point offset; 4. The concentration value of the measured gas is further obtained based on the corresponding relationship between the fringe phase and the gas concentration.

5. The self-calibration method according to claim 4, characterized in that: The line light source (1) is a visible light source with a single wavelength, and the line light source (1) and the photosensitive element (6) are both installed vertically. The light emitted by the line light source (1) generates interference fringes at the photosensitive element (6) after passing through the optical path system.

6. The self-calibration method according to claim 4, characterized in that: The front and rear surfaces of the measuring gas chamber (3) are sealed and isolated from the outside world by high-transmittance plane prisms, and the other directions are sealed and isolated from the outside world by metal. The gas chambers are sealed and isolated by metal, and there is a connecting area between the three reference gas chambers to ensure that the gases in the three reference gas chambers are completely consistent.

7. The self-calibration method according to claim 4, characterized in that: The peak detection algorithm is used to detect the grayscale peak of the received light interference fringes to obtain the effective position for locating the light interference fringes, and the effective positions of the light interference fringes generated by the upper and lower groups of gas chambers after processing are compared to achieve the measurement of gas concentration.

8. The self-calibration method according to claim 4, characterized in that: Preprocess the optical interference signal, including:

1. Grayscale the signal, using gamma transform to map the wider grayscale range in the input image to a narrower grayscale range in the output image, compressing dark pixel values ​​and expanding high grayscale values; 2. Remove noise from the collected image and use median filtering to replace the value of each pixel with the median of its surrounding pixels to remove salt and pepper noise; 3. Extract the optical interference signal after image processing and obtain the peak position of each interference fringe as the comparison position of the optical interference fringes of the upper and lower air chambers.

9. The self-calibration method according to claim 4, characterized in that: The gas concentration detection process is as follows:

1. Fill the standard reference gas into the reference gas chamber from the vent; 2. Open the air inlet of the sampling chamber and place it in the test environment. After the sampling chamber is filled with the gas to be tested, the linear light source (1) emits light and enters the plane mirror (2) with a total reflection film on the back, where it is reflected and refracted at the same time and finally exits the plane mirror (2) as two parallel rays; 3. The two parallel light rays, after passing through the air chamber structure (3), are incident on the prism (4), and are reflected in the light path, and are incident on the air chamber structure (3) again, wherein the light emitted by the line light source (1) is split by the prism (4), passes through the upper standard air chamber and the lower sampling air chamber respectively, and is incident on the plane mirror (2) through the compensation prism (5), and after being reflected on the front and rear surfaces of the plane mirror (2) coated with a total reflection film, interference fringes are generated and are incident on the photosensitive element (6); 4. Combined with the reference interference fringes on the upper part of the photosensitive element (6), the exact concentration of the gas in the sampling chamber can be accurately obtained through the measured light intensity on the lower part of the photosensitive element (6).

10. The self-calibration method according to claim 4, characterized in that: The program algorithm logic of the stripe movement automatic tracking detection is as follows:

1. Perform grayscale conversion on the original signal of the collected stripes, adjust the grayscale range of the signal, and filter the signal to remove the noise mixed in the process of collecting the signal; 2. For the signal after digital image processing, the signal is converted from the time domain to the frequency domain through fast Fourier transform, and the average distribution period T and average phase ψ of the fringe signal are calculated; 3. Based on the fringe period and average phase, the fringe signal in the window is divided into n intervals; 4. Perform extreme value search in each interval to obtain the extreme value of each interval; 5. Compare the extreme values ​​of adjacent intervals, and determine whether they are valid extreme values ​​by comparing the distance of the horizontal axis coordinates, and obtain a new set of valid extreme values; 6. Compare the horizontal coordinates of the effective extreme value with the coordinate position of the previous stripe extreme value, and select the horizontal coordinates of the two extreme values ​​with the closest Euclidean distance; 7. According to the first five values ​​of the fringe extreme value coordinates, a polynomial fitting curve is used to compare the changing trend, and the value with the minimum interpolation of the curve is selected as the effective fringe position for measurement.

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