A gas detection system and method based on a chopper dual filter

Abstract

This invention discloses a gas detection system and method based on a chopper and dual filters, belonging to the field of optical gas detection technology. Its purpose is to solve the problems of existing optical gas detection technologies, such as the difficulty in balancing detection sensitivity and response speed, noise introduced by poor consistency between dual detectors, and low stability due to complex mechanical structures. It includes a light source, a chopper, a gas chamber, a detector, and a signal processor arranged sequentially. The chopper has two through-holes, each containing a detector wavelength filter and a reference wavelength filter. As the chopper rotates, the detector and reference wavelength filters alternately pass through the gas chamber. Light emitted from the light source passes sequentially through the filters on the chopper and the gas chamber before being focused onto the detector, which outputs a signal to the signal processor. By using a single detector to receive both the reference and detector light, the noise introduced by inconsistent temperature characteristics and aging levels of dual detectors is fundamentally eliminated.

Description

Technical Field

[0001] This invention belongs to the field of optical gas detection technology, and relates to high signal-to-noise ratio gas detection, and particularly to a gas detection system and method based on a chopper dual filter. Background Technology

[0002] Gas detection technology based on gas absorption spectroscopy offers advantages over traditional electrochemical, semiconductor compound, and catalytic combustion gas detection technologies, including excellent selectivity, long lifespan, long-range operation, non-contact operation, and high reliability, making it an important direction for the development of gas detection technology. However, when extremely high detection accuracy is required (e.g., ppm or ppb detection sensitivity), optical gas detection technology often suffers from inherent noise, such as light source fluctuations and detector signal drift, as well as environmental interference, such as signal fluctuations caused by changes in temperature, humidity, and air pressure, which severely affect the detection results and fail to meet application requirements. Therefore, effectively suppressing signal fluctuations to obtain high signal-to-noise ratio detection results is crucial for the development of ultra-high sensitivity optical gas sensors. However, in practical applications, existing gas detection technologies still have many shortcomings, making it difficult to simultaneously meet the requirements of high sensitivity, high stability, compact structure, and wide measurement range.

[0003] Patent application No. 202411165941.0 discloses a gas detection method based on a multi-channel integrating sphere optical cell. Although the integrating sphere increases the optical path, it switches filter channels by mechanically rotating the integrating sphere, which is relatively complex. Moreover, multiple diffuse reflections of the integrating sphere will cause significant attenuation of light energy (attenuation rate can reach 40%-60%), which limits the detection lower limit. At the same time, this technology is prone to signal saturation due to excessively long optical path when detecting high-concentration gases, making it difficult to detect both high and low concentrations.

[0004] The invention patent application with application number 202411332262.8 discloses a gas online detection method and device based on light source frequency modulation. It adopts light source frequency modulation and algorithm inversion. Although it simplifies the filter structure, it is highly dependent on the complex algorithm model, has weak anti-interference ability, and requires retraining of the model when the environment changes. It is difficult to maintain and the system stability is greatly affected by the robustness of the algorithm.

[0005] The invention patent application with application number 202311675951.4 discloses a low-interference SF6 decomposition gas photoacoustic spectroscopy detection device and detection method. It achieves range switching through complex mechanical rotation and aperture adjustment, which not only has a high failure rate, but also cannot achieve true synchronous detection, and is bulky and difficult to miniaturize.

[0006] The invention patent application with application number 202411417780.X discloses a laser gas telemetry imaging system and detection method based on micro-mirror scanning. Although it is suitable for large-scale monitoring, it has low quantitative accuracy, is easily affected by weather and ambient light interference, and has high equipment cost, making it unsuitable for low-cost, high-precision fixed-point detection.

[0007] Furthermore, traditional NDIR (non-dispersive infrared) gas detection technology often employs dual detectors—a detection channel plus a reference channel—to eliminate light source fluctuations. However, since the temperature characteristics and response nonlinearities of the two detectors cannot be completely identical, this inconsistency introduces significant common-mode noise in high-precision detection at the ppm or even ppb level. Literature shows that dual-detector NDIR systems experience temperature drift of ±3%-5% when the temperature changes by 20°C, severely impacting detection accuracy. While GFC (gas correlation filter) technology offers high accuracy, it requires a mechanical structure to rotate two gas chambers. The relatively large size of these chambers makes it difficult to achieve high rotation frequencies, resulting in a lower response speed and making it unsuitable for scenarios with rapid concentration changes. Simultaneously, the gas chambers are sensitive to fluctuations in ambient temperature and humidity; temperature changes alter the gas refractive index and window transmittance, introducing measurement errors. Moreover, industrial vibrations can easily cause sensor optical path misalignment and increased signal noise, requiring additional vibration damping and temperature control, increasing system complexity and cost.

[0008] Therefore, there is an urgent need for a gas detection device and method that can detect both high and low concentrations, has a compact structure without complex mechanical movements, and maintains high sensitivity and high stability. Summary of the Invention

[0009] The purpose of this invention is to provide a gas detection system and method based on a chopper dual filter, in order to solve the problems in existing optical gas detection technologies, such as the difficulty in balancing detection sensitivity and response speed, the introduction of noise due to poor consistency between dual detectors, and the low stability caused by complex mechanical structures.

[0010] To achieve the above objectives, the present invention specifically adopts the following technical solution: A gas detection system based on a chopper and dual filters includes a light source, a chopper, a gas chamber, a detector, and a signal processor arranged sequentially. The chopper has two through holes, and a detection wavelength filter and a reference wavelength filter are respectively arranged in the two through holes. The chopper rotates, and the detection wavelength filter and the reference wavelength filter alternately pass through the gas chamber. The light emitted by the light source passes through the filter and air cell on the chopper in sequence and is then focused onto the detector. The detector outputs a signal to the signal processor.

[0011] A gas detection system based on a chopper and dual filters includes a light source, a convex lens, a concave mirror, a diffuse reflective surface, a chopper, a detector, and a signal processor arranged sequentially. The chopper has two through holes, and a detection wavelength filter and a reference wavelength filter are respectively arranged in the two through holes. The chopper rotates, and the detection wavelength filter and the reference wavelength filter alternately rotate through the gas chamber. The light emitted from the light source passes through a convex lens and a concave mirror in sequence before being incident on a diffuse reflection surface and reflected. The reflected light is then incident on a filter on a chopper and focused onto a detector, which outputs a signal to a signal processor.

[0012] A gas detection method based on a chopper dual-filter includes the following steps: Step 1: Construct a gas detection system; Construct the above-mentioned gas detection system based on a chopper dual filter; Step 2: Time-sharing data collection; The chopper rotates at a set frequency, and the detection wavelength filter and the reference wavelength filter alternately rotate through the air chamber; the light emitted by the light source alternately passes through the wavelength filter or the reference wavelength filter and is then received by the detector, which outputs the raw time-domain waveform containing environmental fluctuation information. Step 3: Time-division differential processing; The signal peaks of the corresponding probe wavelength light and reference wavelength light in the original time domain waveform are extracted respectively, and the differential signal is obtained based on the probe signal peak and the reference signal peak. Step 4: Concentration inversion; Indoor gas concentration is calculated by using the amplitude inversion of differential signals.

[0013] Furthermore, in step 2, the rotation frequency of the chopper is 5~200Hzs.

[0014] Furthermore, in step 3, the differential signal is subjected to mean filtering or Kalman filtering, and the filtered differential signal is used to calculate the indoor gas concentration.

[0015] Furthermore, the differential signal is subjected to Kalman filtering. The state equation for Kalman filtering is: ; The observation equation is: ; in, This represents the state vector at time k. Represents the state transition matrix. Denotes the observation vector at time k. Represents the observation matrix. Indicates process noise. This indicates observation noise.

[0016] Furthermore, in step 4, the indoor gas concentration is calculated using the amplitude inversion of the differential signal. The calculation formula is as follows: ; in, Indicates gas concentration. Indicates the amplitude of the reference filter channel signal. Indicates the amplitude of the detection filter channel signal. Representation function The inversion function.

[0017] The beneficial effects of this invention are as follows: 1. High stability and high sensitivity: Employing a single detector to receive both reference and probe light fundamentally eliminates the common-mode noise introduced by dual detectors due to inconsistent temperature characteristics and aging processes. Experimental data shows that compared to dual-detector NDIR systems, the single-detector time-division differential scheme reduces temperature drift from ±3%-5% to within ±0.5% within a 20℃ temperature range, improving the signal-to-noise ratio by 10-15dB. This significantly enhances the signal-to-noise ratio, making it suitable for long-distance detection of trace amounts of common infrared-absorbing gases such as carbon dioxide, methane, and carbon monoxide.

[0018] 2. Balancing high and low concentrations: The light source intensity fluctuation is calibrated in real time through the reference channel, and the differential algorithm can effectively suppress baseline drift. This allows the system to maintain linearity and accuracy under both high concentrations (large signal attenuation) and low concentrations (small signal changes), avoiding the saturation problem of integrating sphere and other techniques at high concentrations. Test results show that the detection limit can reach the ppb level, and the dynamic range can reach 4 orders of magnitude.

[0019] 3. Compact structure and no complex mechanical movement: Compared with the complex rotating structure in the prior art, the present invention only requires a simple dual-hole chopper (or galvanometer switching), without complex integrating sphere rotation or aperture adjustment mechanism, and has a small size and low failure rate.

[0020] 4. Wide applicability: It is suitable for both NDIR detection in the infrared band and DOAS detection in the ultraviolet band; it can be used for both inhalation gas chamber detection and long-distance telemetry; and by adding filters corresponding to the absorption wavelengths of different gases, the number of gases that can be detected at the same time can be increased.

[0021] 5. Significant cost advantages: Compared with dual-detector NDIR technology, the single-detector configuration reduces detector costs by 50% and overall sensor costs by 30%-40%. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the gas detection system in Embodiment 1 of the present invention; Figure 2 This is a schematic diagram of the gas detection system in Embodiment 3 of the present invention; Figure 3 This is a schematic diagram of the original waveform of the signal amplitude received by the detector in this invention; Figure 4 This is a schematic diagram of the difference between the maximum value of the probe signal and the reference signal before filtering in this invention; Figure 5 This is a schematic diagram of the filtered difference between the maximum values ​​of the probe signal and the reference signal in this invention; Figure 6 This is a schematic diagram of the gas detection system in Embodiment 2 of the present invention; The attached figures are labeled as follows: 1-Detection wavelength filter, 2-Reference wavelength filter, 3-Chopper, 4-Gas chamber, 5-Light source, 6-Detector, 7-Convex lens, 8-Concave mirror, 9-Diffuse reflection surface, 10-Signal processor, 11-First filter, 12-Second filter, 13-Third filter, 14-Fourth filter. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments.

[0024] Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0025] Example 1 This embodiment provides a gas detection system based on a chopper dual-filter, which can be used for close-range (inhalation) detection of trace amounts of carbon dioxide. For example... Figure 1 As shown, the gas detection system includes a light source 5, a chopper 3, a gas chamber 4, a detector 6, and a signal processor 10 arranged in sequence; the chopper 3 has two through holes, and a detection wavelength filter 1 and a reference wavelength filter 2 are respectively arranged in the two through holes; when the chopper 3 rotates, the detection wavelength filter 1 and the reference wavelength filter 2 alternately rotate through the gas chamber 4; The light emitted by the light source 5 passes through the filter on the chopper 3 and the air chamber 4 in sequence before being focused onto the detector 6. The detector 6 outputs a signal to the signal processor 10.

[0026] Light source 5 is an infrared light source with a wavelength coverage of 2-7μm. Chopper 3 is driven by a micro-motor with an adjustable speed of 10-50Hz, allowing it to rotate at a set frequency. The detector wavelength filter 1, mounted on the first through-hole of chopper 3, has a center wavelength of 4.26μm, corresponding to the CO2 absorption peak, a full width at half maximum (FWHM) of 15nm, and a center wavelength accuracy of ±0.5nm. The reference wavelength filter 2, mounted on the second through-hole of chopper 3, has a center wavelength of 3.93μm, corresponding to a non-absorption window, and a FWHM of 20nm. Gas chamber 4 is 10cm long and has light-transmitting windows (made of calcium fluoride with an infrared transmittance ≥95%) at both ends. Detector 6 is a pyroelectric infrared detector with a noise equivalent power (NEP). It is located behind air chamber 4.

[0027] When using this gas detection system for gas detection, the gas detection method includes the following steps: Step 1: Construct a gas detection system; Construct the gas detection system based on a chopper dual filter as described above; Step 2: Time-sharing data collection; The chopper 3) rotates at a set frequency, and the detection wavelength filter 1 and the reference wavelength filter 2 alternately rotate through the air chamber 4; the light emitted by the light source 5 is received by the detector 6 after alternately passing through the wavelength filter 1 or the reference wavelength filter 2, and the detector 6 outputs the original time domain waveform containing environmental fluctuation information. The rotation frequency of chopper 3 is 5~200Hz.

[0028] Step 3: Time-division differential processing; The signal peaks of the corresponding probe wavelength light and reference wavelength light in the original time domain waveform are extracted respectively, and the differential signal is obtained based on the probe signal peak and the reference signal peak. The differential signal can be obtained by directly subtracting or dividing the peak value of the probe signal and the peak value of the reference signal within the same (or adjacent) period. The calculation formula is as follows: ; or ; In addition, the differential signal can be processed by mean filtering or Kalman filtering, and the filtered differential signal can be used to calculate the indoor gas concentration. If Kalman filtering is used on the differential signal, the state equation of Kalman filtering is: ; The observation equation is: ; in, This represents the state vector at time k. Represents the state transition matrix. Denotes the observation vector at time k. Represents the observation matrix. Indicates process noise. This indicates observation noise.

[0029] Step 4: Concentration inversion; Indoor gas concentration is calculated using amplitude inversion of differential signals: that is, based on... ; achievable ; in, Indicates gas concentration. Indicates the amplitude of the reference filter channel signal. This indicates the amplitude of the signal in the detection filter channel. This represents any function, determined by fitting calibration data, and is generally a power polynomial or exponential function. express function The inverse function.

[0030] In this embodiment, when the chopper 3 rotates at a set frequency, the waveform of the signal amplitude received by the detector 6 is as follows: Figure 3 As shown. By Figure 3 It can be seen that the odd and even half-wave waveforms are the detector signals when the light intensity is transmitted through the reference filter and the detector filter, respectively. The maximum values ​​of 1, 1', 2, 2', etc. represent the signal amplitudes when the centers of the two filters are directly opposite the center of the light source (assuming that the light intensity of the light spot emitted from the light source is Gaussian distributed). Figure 3 It also illustrates the effect of changes in environmental conditions (such as temperature) on the signal, that is, the baseline of the entire waveform amplitude shifts upward over time.

[0031] Considering that the chopping rate is much greater than the rate of change of the waveform over time caused by changes in environmental conditions, only one data point needs to be taken from all the data points of the half-wave waveform corresponding to each filter; here, we take its maximum value. Subtracting the maximum values ​​corresponding to the two filters yields the result. Figure 4 As shown in the waveform, it can be seen that since the baseline drift of the waveform amplitude has an almost identical effect on the two filters, the fluctuation of the maximum value difference is only random noise. Figure 5 As shown, by applying mean filtering or other filtering methods (such as Kalman filtering), most of the random noise can be easily filtered out. The resulting noise fluctuation ( Figure 5 (Compared) Figure 3 It will be significantly reduced.

[0032] Example 2 This embodiment provides a gas detection system based on a chopper with four filters, which can be used for close-range (inhalation) detection of trace amounts of various gases such as carbon dioxide, methane, and carbon monoxide.

[0033] like Figure 6 As shown, the gas detection system includes a light source 5, a chopper 3, a gas chamber 4, a detector 6, and a signal processor 10 arranged sequentially. The chopper 3 has four through-holes, each housing a first filter 11 with a center wavelength of 4.26 μm, a second filter 12 with a center wavelength of 3.31 μm, a third filter 13 with a center wavelength of 4.66 μm, and a fourth filter 14 with a center wavelength of 3.39 μm. The first filter 11, second filter 12, and third filter 13 are all detection wavelength filters, corresponding to carbon dioxide, methane, and carbon monoxide gases, respectively; the fourth filter 14 is a reference filter. When the chopper 3 rotates, the four filters alternately pass through the gas chamber 4.

[0034] The light emitted by the light source 5 passes through the filter on the chopper 3 and the air chamber 4 in sequence before being focused onto the detector 6. The detector 6 outputs a signal to the signal processor 10.

[0035] Light source 5 is an infrared light source with a wavelength coverage of 2-7μm. Chopper 3 is driven by a micro-motor with an adjustable speed of 10-50Hz, allowing it to rotate at a set frequency. Gas chamber 4 is 10cm long and has light-transmitting windows (made of calcium fluoride with an infrared transmittance ≥95%) at both ends. Detector 6 is a pyroelectric infrared detector with a noise equivalent power (NEP). It is located behind air chamber 4.

[0036] When using this gas detection system for gas detection, the gas detection method is the same as the detection steps in Example 1, and will not be described again in this example; those skilled in the art can refer to the detection steps in Example 1 to implement it without any creative effort.

[0037] Example 3 This embodiment is used for long-distance detection in the infrared band. For example... Figure 2 As shown, it includes a light source 5, a convex lens 7, a concave mirror 8, a diffuse reflection surface 9, a chopper 3, a detector 6, and a signal processor 10 arranged in sequence; the chopper 3 has two through holes, and a detection wavelength filter 1 and a reference wavelength filter 2 are respectively arranged in the two through holes; the chopper 3 rotates, and the detection wavelength filter 1 and the reference wavelength filter 2 alternately rotate through the air chamber 4; The light emitted by the light source 5 passes through the convex lens 7 and the concave mirror 8 in sequence and then enters the diffuse reflection surface 9 to be reflected. The reflected light enters the filter on the chopper 3 and is focused onto the detector 6. The detector 6 outputs a signal to the signal processor 10.

[0038] When using this gas detection system for gas detection, the gas detection method is the same as the detection steps in Example 1, and will not be described again in this example; those skilled in the art can refer to the detection steps in Example 1 to implement it without any creative effort. The difference lies in the inversion calculation formula.

[0039] For long-distance detection of leaked toxic gases in the air, since the gas diffuses in the air and has no fixed concentration or length, only the following inversion formula can be obtained from the signal to be measured: ; Where C and L are the equivalent concentration and equivalent length of the leaked gas in the air, respectively. That is, detecting leaked gas in the air can only provide information in ppm·m (equivalent gas concentration multiplied by effective length) and not the specific gas concentration.

Claims

1. A gas detection system based on a chopper dual-filter, characterized in that: It includes a light source (5), a chopper (3), an air chamber (4), a detector (6), and a signal processor (10) arranged in sequence; the chopper (3) has two through holes, and a detection wavelength filter (1) and a reference wavelength filter (2) are respectively arranged in the two through holes; the chopper (3) rotates, and the detection wavelength filter (1) and the reference wavelength filter (2) alternately rotate through the air chamber (4); The light emitted by the light source (5) passes through the filter on the chopper (3) and the air chamber (4) in sequence and is focused onto the detector (6). The detector (6) outputs a signal to the signal processor (10).

2. A gas detection system based on a chopper dual-filter, characterized in that: The system includes a light source (5), a convex lens (7), a concave mirror (8), a diffuse reflection surface (9), a chopper (3), a detector (6), and a signal processor (10) arranged in sequence. The chopper (3) has two through holes, and a detection wavelength filter (1) and a reference wavelength filter (2) are respectively arranged in the two through holes. When the chopper (3) rotates, the detection wavelength filter (1) and the reference wavelength filter (2) alternately rotate through the air chamber (4). The light emitted by the light source (5) passes through the convex lens (7) and the concave mirror (8) in sequence and then enters the diffuse reflection surface (9) to produce reflection. The reflected light enters the filter on the chopper (3) and is focused onto the detector (6). The detector (6) outputs a signal to the signal processor (10).

3. A gas detection method based on a chopper dual-filter, characterized in that, Includes the following steps: Step 1: Construct a gas detection system; Construct a gas detection system based on a chopper dual filter as described in claim 1; Step 2: Time-sharing data collection; The chopper (3) rotates at a set frequency, and the detection wavelength filter (1) and the reference wavelength filter (2) alternately rotate through the air chamber (4); the light emitted by the light source (5) alternately passes through the wavelength filter (1) or the reference wavelength filter (2) and is received by the detector (6), and the detector (6) outputs the original time-domain waveform containing environmental fluctuation information. Step 3: Time-division differential processing; The signal peaks of the corresponding probe wavelength light and reference wavelength light in the original time domain waveform are extracted respectively, and the differential signal is obtained based on the probe signal peak and the reference signal peak. Step 4: Concentration inversion; Indoor gas concentration is calculated by using the amplitude inversion of differential signals.

4. The gas detection method based on a chopper dual filter as described in claim 3, characterized in that, In step 2, the rotation frequency of the chopper (3) is 5~200Hzs.

5. The gas detection method based on a chopper dual filter as described in claim 3, characterized in that, In step 3, the differential signal is processed by mean filtering or Kalman filtering, and the filtered differential signal is used to calculate the indoor gas concentration.

6. The gas detection method based on a chopper dual filter as described in claim 3, characterized in that, The state equation for Kalman filtering of differential signals is as follows: ； The observation equation is: ； in, This represents the state vector at time k. Represents the state transition matrix. Denotes the observation vector at time k. Represents the observation matrix. Indicates process noise. This indicates observation noise.

7. The gas detection method based on a chopper dual filter as described in claim 3, characterized in that, In step 4, the indoor gas concentration is calculated using the amplitude inversion of the differential signal. The calculation formula is as follows: ； in, Indicates gas concentration. Indicates the amplitude of the reference filter channel signal. Indicates the amplitude of the detection filter channel signal. Representation function The inversion function.

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