A calibration-free harmonic signal demodulation method based on logarithmic method

Abstract

The present invention discloses a calibration-free harmonic signal demodulation method based on the logarithmic method, in which the gas absorption spectrum signal is taken logarithmically and then the harmonic signal is obtained through phase-locked filtering. The present invention effectively suppresses the interference of the light intensity signal on the harmonic signal linearly related to the gas concentration through a relatively simple method. Compared with the traditional phase-locked amplification method, the step of calibrating the light intensity signal is omitted, the operation is simpler, the calculation speed is faster, the resource occupation is less, and the application range is wider.

Description

Technical Field

[0001] The invention relates to the technical field of gas detection, and in particular to a calibration-free harmonic signal demodulation method based on a logarithmic method. Background Art

[0002] At present, Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology has been widely used in the field of carbon dioxide detection due to its advantages of non-contact, strong selectivity, high sensitivity, and fast online response speed. Among them, Wavelength Modulation Spectroscopy (WMS) technology is often used in combination with TDLAS technology due to its extremely high noise suppression ability. The basic principle of TDLAS-WMS technology to measure gas concentration is based on the linear relationship between the multiple harmonic components of the absorption spectrum signal and the gas concentration. Therefore, accurately demodulating the multiple harmonics from the absorption spectrum signal is an important guarantee for calculating the gas concentration.

[0003] However, TDLAS-WMS technology usually uses a ramp signal and a cosine signal to tune the laser at the same time. The ramp signal determines the central light intensity of the laser. Therefore, directly performing phase-locked amplification on the collected absorption light intensity signal will obtain a harmonic signal containing the central light intensity component. The change in the central light intensity will cause the harmonic signal to deform. In order to eliminate the influence of the central light intensity on the harmonic signal, it is necessary to determine the central light intensity of the laser before the experiment for subsequent calibration of the harmonic signal. However, since environmental factors have a great influence on the central light intensity, this method has poor anti-interference performance. In experiments, a calibration-free method is often used to eliminate the influence of the central light intensity.

[0004] The traditional calibration-free method uses the first harmonic signal to normalize other harmonic signals, which is the "nf / 1f" method. However, the "nf / 1f" method will introduce the RAM signal existing in the first harmonic during measurement, so it is necessary to perform additional calibration on the existing RAM signal before measurement, which will increase the complexity and difficulty of the experiment. When the optical path drifts, the RAM signal may also cause measurement errors. In addition, the "nf / 1f" method can only calibrate the amplitude at the center of the extracted harmonic absorption peak, and cannot calibrate the complete spectrum.

[0005] An improved calibration-free method is to use the zero-order harmonic to normalize other harmonic signals, namely the "nf / 0f" method. When the gas absorbance is small, the zero-order harmonic can approximately replace the central light intensity I0, so the zero-order harmonic can be used to calibrate other harmonics. The "nf / 0f" method does not need to consider the impact of the RAM signal on harmonic extraction, nor will it introduce large measurement errors, and it can calibrate all amplitudes of the complete spectrum of the extracted harmonics. However, when the gas absorbance is large, the central light intensity cannot be approximately replaced by the zero-order harmonic, and the "nf / 0f" method will produce a large error and fail. Summary of the invention

[0006] In view of this, the present invention proposes to solve the problem in the prior art that when the gas absorption degree is large, the measurement error using the "nf / 0f" method is large and may lead to failure.

[0007] To this end, the specific technical solution adopted by the present invention is as follows:

[0008] The calibration-free harmonic signal demodulation method based on logarithmic method comprises the following steps:

[0009] Step 1: Build the TDLAS-WMS gas detection system;

[0010] Step 2, collecting gas absorption spectrum signals;

[0011] Step 3, performing logarithmic processing on the gas molecule absorption spectrum signal;

[0012] Step 4, performing phase-locked filtering on the gas molecule absorption spectrum signal after taking the logarithm in step 3 to obtain the kth harmonic signal linearly related to the concentration information.

[0013] In a possible design, step 2 is specifically, according to the Beer-Lambert law, the absorption spectrum signal I τ (t) is expressed as follows:

[0014]

[0015] Where v is the wave number and α(v) is the absorbance, which can be expressed as Where P is the total atmospheric pressure, S(T) is the spectral line intensity, C is the gas concentration, and L is the absorption path length. is the absorption line profile, H k for is the k-order Fourier coefficient of , and I0(t) is the incident light intensity.

[0016] In one possible design, when the laser is modulated by a high-frequency cosine signal, the incident light intensity I0(t) is expressed as follows:

[0017]

[0018] in, is the central light intensity determined by the ramp signal, I k is the k-order cosine modulation intensity, ω is the modulation frequency, φ k is the phase shift between wavelength modulation and k-th order intensity modulation. For a given modulation frequency ω, φ k is fixed, definition: θ=ωt+η,

[0019] In a possible design, step 3 is specifically to perform a logarithmic operation on the collected absorption spectrum signal, as shown in the following formula:

[0020]

[0021] In a possible design, step 3 also includes approximating the ln[I0(t)] part, which is shown in the following formula:

[0022]

[0023]

[0024] In a possible design, in step 4, the absorption spectrum signal after logarithm is taken and multiplied and filtered by the doubled frequency reference signal to obtain the second harmonic signal, as shown in the following formula:

[0025]

[0026] In one possible design, i2 and The two parts can be approximated as the intercept and slope of the signal deviation in the subsequent concentration calculation, and PS(T)CLH2 is the second harmonic signal related to the concentration.

[0027] In a possible design, during the phase-locked filtering process, the frequency-multiplied reference signal is changed to obtain a k-th order calibration-free harmonic signal.

[0028] The present invention also provides a calibration-free harmonic signal demodulation device based on the logarithmic method, comprising a memory, a control processor, and a computer program stored in the memory and executable on the control processor. The control processor executes the program to implement the aforementioned calibration-free harmonic signal demodulation method based on the logarithmic method.

[0029] The present invention also provides a computer-readable storage medium, wherein the computer-readable storage medium stores computer-executable instructions, and the computer-executable instructions are used to enable a computer to execute the aforementioned calibration-free harmonic signal demodulation method based on the logarithmic method.

[0030] Compared with the prior art, the present invention has the following beneficial effects: the method is fast and accurate, can realize phase-locked demodulation of absorption spectrum signals, and provides technical support for the analysis of trace gases. Compared with the "nf / 0f" calibration-free method, the present method does not require additional calculation of zero-order harmonics, and is not limited to the premise that the absorbance of the gas to be measured is low. The calculation speed is faster, the resource consumption is less, and the application range is wider. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For those skilled in the art, other drawings can be obtained based on these drawings without creative work.

[0032] Figure 1 A flow chart of an embodiment of a calibration-free harmonic signal demodulation method based on a logarithmic method of the present invention is shown;

[0033] Figure 2 A comparison diagram of the second harmonic demodulated in the embodiment of the calibration-free harmonic signal demodulation method based on the logarithmic method of the present invention and the second harmonic demodulated based on the "nf / 0f" calibration-free method is shown. DETAILED DESCRIPTION

[0034] To further illustrate each embodiment, the present invention provides drawings, which are part of the disclosure of the present invention and are mainly used to illustrate the embodiments and can be used in conjunction with the relevant descriptions in the specification to explain the operating principles of the embodiments. With reference to these contents, ordinary technicians in the field should be able to understand other possible implementations and advantages of the present invention. The components in the figures are not drawn to scale, and similar component symbols are generally used to represent similar components.

[0035] The calibration-free harmonic signal demodulation method based on the logarithmic method provided in this example comprises the following steps:

[0036] Step 1: Build a gas detection system based on TDLAS-WMS technology, use carbon dioxide standard gas as the gas to be detected to verify the effectiveness of the system, and use the superposition signal of low-frequency ramp signal and high-frequency cosine signal as the current modulation signal of the laser. Under this modulation mode, the incident light intensity I0(t) of the laser is shown as follows:

[0037]

[0038] in, is the central light intensity determined by the ramp signal, I k is the k-order sinusoidal modulation intensity, ω is the modulation frequency, φ kis the phase shift between wavelength modulation and k-th order intensity modulation. For a given modulation frequency ω, φ k is fixed, definition: θ=ωt+η,

[0039] Step 2: Collect gas absorption spectrum signals. According to the Beer-Lambert law, the absorption spectrum signal I τ (t) is expressed as follows:

[0040]

[0041] Where v is the wave number and α(v) is the absorbance, which can be expressed as Where P is the total atmospheric pressure, S(T) is the spectral line intensity, C is the gas concentration, and L is the absorption path length. is the absorption line profile, H k for The k-th order Fourier coefficient of .

[0042] Step 3, perform logarithmic operation on the collected absorption spectrum signal, as shown in the following formula:

[0043]

[0044] The ln[I0(t)] part is approximated as shown below:

[0045]

[0046] Step 4, multiply the absorption spectrum signal after taking the logarithm by the double frequency reference signal to filter out the second harmonic component, and the result is shown in the following formula:

[0047]

[0048]

[0049] The filtering results are i2 and The two parts can be approximated as the intercept and slope of the signal offset in subsequent calculations. PS(T)CLH2 is the second harmonic signal related to the concentration. In the phase-locked filtering process, the k-th order calibration-free harmonic signal can be obtained by changing the frequency-doubled reference signal.

[0050] The absorption spectrum signal of the collected carbon dioxide standard gas was extracted using this method and the "nf / 0f" calibration-free method. The results are as follows: Figure 2 As shown in the figure, the second harmonic extracted by this method and the "nf / 0f" calibration-free method are basically consistent at the absorption peak, but there is a certain deviation at the baseline. The reason is that the second harmonic extracted by this method contains i2 and components, which can be approximated by the intercept and slope of the signal shift and can be eliminated in subsequent concentration calculations.

[0051] In this example, only the second harmonic is extracted for verification. By changing the selected frequency-multiplied reference signal, calibration-free harmonic signals of other orders can be obtained.

[0052] The present invention also provides a calibration-free harmonic signal demodulation device based on the logarithmic method, comprising a memory, a control processor, and a computer program stored in the memory and executable on the control processor. The control processor executes the program to implement the aforementioned phase-locked amplification method.

[0053] The present invention also provides a computer-readable storage medium, wherein the computer-readable storage medium stores computer-executable instructions, and the computer-executable instructions are used to enable a computer to execute the aforementioned phase-locked amplification method. Examples of computer-readable storage media include: read-only memory (ROM), random access programmable read-only memory (PROM), electrically erasable programmable read-only memory (EEPROM), random access memory (RAM), dynamic random access memory (DRAM), static random access memory (SRAM), flash memory, non-volatile memory, CD-ROM, CD-R, CD+R, CD-RW, CD+RW, DVD-ROM, DVD-R, DVD+R, DVD-RW, DVD+RW, DVD-RAM, BD-ROM, BD-R, BD-R LTH, BD-RE, Blu-ray or optical disk storage, hard disk drive (HDD), solid state drive (SSD), card storage (such as, MultiMediaCard, Secure Digital (SD) card or Extreme Digital (XD) card), magnetic tape, floppy disk, magneto-optical data storage device, optical data storage device, hard disk, solid state disk and any other device, the any other device is configured to store the computer program and any associated data, data files and data structures in a non-transitory manner and provide the computer program and any associated data, data files and data structures to a processor or computer so that the processor or computer can execute the computer program.

[0054] In one example, a computer program and any associated data, data files, and data structures are distributed across networked computer systems so that the computer program and any associated data, data files, and data structures are stored, accessed, and executed in a distributed fashion by one or more processors or computers.

[0055] This method is fast and accurate, and can realize phase-locked demodulation of absorption spectrum signals, providing technical support for the analysis of trace gases. Compared with the "nf / 0f" calibration-free method, this method does not require additional calculation of zero-order harmonics, and is not limited to the premise of low absorbance of the gas to be measured. It has faster calculation speed, less resource consumption, and a wider range of applications.

[0056] Although the above methods are illustrated and described as a series of actions for simplicity of explanation, it should be understood and appreciated that these methods are not limited by the order of the actions, because according to one or more embodiments, some actions may occur in a different order and / or concurrently with other actions from those illustrated and described herein or not illustrated and described herein but understandable to those skilled in the art.

[0057] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A calibration-free harmonic signal demodulation method based on logarithmic method, characterized in that: The following steps are included: Step 1: Build the TDLAS-WMS gas detection system; Step 2, collecting gas absorption spectrum signal; Step 2 specifically includes, according to the Beer-Lambert law, the absorption spectrum signal I τ (t) is expressed as follows: ； Among them, ν is the wave number, α(ν) is the absorbance, and its expression is , where P is the total atmospheric pressure, S(T) is the spectral line intensity, C is the gas concentration, and L is the absorption path length. is the absorption line profile, H k for The k-order Fourier coefficient of , I0(t) is the incident light intensity; when the laser is modulated by a high-frequency cosine signal, the incident light intensity I0(t) is as follows: ； in, is the central light intensity determined by the ramp signal, I k is the k-order cosine modulation intensity, ω is the modulation frequency, is the phase shift between wavelength modulation and k-th order intensity modulation. For a given modulation frequency ω, is fixed, definition: θ=ωt+η, ; Step 3, performing logarithmic processing on the gas molecule absorption spectrum signal; Step 3 specifically includes performing logarithmic operation on the collected absorption spectrum signal, as shown in the following formula: ； Step 3 also includes approximating the ln[I0(t)] part, which is shown in the following formula: ； Step 4, phase-locked filtering is performed on the gas molecule absorption spectrum signal after taking the logarithm in step 3 to obtain the kth harmonic signal which is linearly related to the concentration information. In step 4, the absorption spectrum signal after taking the logarithm is multiplied and filtered by the double frequency reference signal to obtain the second harmonic signal, as shown in the following formula: ； i2 and The two parts are approximately the intercept and slope of the signal offset in the subsequent concentration calculation. PS(T)CLH2 is the second harmonic signal related to the concentration. In the phase-locked filtering process, the k-th order calibration-free harmonic signal can be obtained by changing the frequency-doubled reference signal.

2. A calibration-free harmonic signal demodulation device based on a logarithmic method, characterized in that: The invention comprises a memory, a control processor and a computer program stored in the memory and executable on the control processor, wherein the control processor executes the program to implement the calibration-free harmonic signal demodulation method based on the logarithmic method as claimed in claim 1.

3. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores computer-executable instructions, and the computer-executable instructions are used to enable a computer to execute the calibration-free harmonic signal demodulation method based on the logarithmic method as claimed in claim 1.

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