Absorption peak image feature construction and matching absorption spectrum gas velocity measurement method

Abstract

The invention discloses an absorption spectrum gas velocity measurement method based on absorption peak image feature construction and matching, which comprises the following steps: performing multiple phase shift down-sampling on a detected wavelength modulation laser signal, and reconstructing a multiple phase shift sequence to obtain a two-dimensional grayscale image matrix containing a direct absorption spectrum profile; sharpening the two-dimensional grayscale image matrix extracted from the laser signal with the flow field velocity being zero to obtain an image template representing the position of an absorption peak; according to the image template of the absorption peak position, matching the position of the image template in a two-dimensional grayscale image obtained from the laser detection signals in the two directions in the flow field to be measured, and determining the offset of the absorption peak position in the two directions; and the airflow velocity is calculated by using the absorption peak offset in the two detection directions. By adopting the technical scheme of the invention, the absorption spectrum frequency shift solving precision under different optical depths is improved, and the speed measurement precision in a complex flow field with changed working conditions is further improved.

Description

Technical Field

[0001] The invention belongs to the technical field of gas parameter measurement, and in particular relates to an absorption spectrum gas velocity measurement method by constructing and matching absorption peak image features. Background Art

[0002] Rocket and aerospace engines rely on continuous design and validation cycles for continuous updates. Accurate measurements of gas velocity and temperature are critical to accelerating this iterative process. Currently, the measurement of these parameters mainly uses contact methods, such as using thermocouples to measure temperature and using pitot tubes to measure gas velocity. The contact method requires the probe to contact the flow field and sense the parameters, which will affect the original flow field and result in a slow response. In recent years, many non-contact combustion monitoring methods have been developed for measuring temperature and gas velocity in high-temperature and high-speed flow fields. Among the many non-contact methods, TDLAS (tunable diode laser absorption spectroscopy) technology has unique advantages in fast resolution, quantitative measurement and low cost. TDLAS achieves simultaneous measurement of multiple gas parameters, including gas concentration, temperature, velocity and pressure, by measuring the absorption spectrum of specific molecules in the flow field.

[0003] In terms of velocity measurement, TDLAS technology measures gas velocity by acquiring the Doppler frequency shift of the molecular absorption spectrum in the flow field. Compared with other non-contact measurement methods such as PIV (particle image velocimetry) and KTV (krypton marker velocimetry), TDLAS technology uses existing gas molecules in the flow field as tracer molecules without the need for additional tracer molecules or particles. Unlike LDV (laser Doppler velocimetry)

[18] , which measures velocity from scattered laser signals, velocity measurement based on TDLAS is achieved by extracting from the transmission signal, which has a higher signal-to-noise ratio. Therefore, TDLAS is considered to be a very promising non-invasive, high-precision flow velocity measurement technology.

[0004] TDLAS measurement methods include: DAS (direct absorption spectroscopy) and WMS (wavelength modulation spectroscopy). The DAS method uses a linear wavelength scanning laser to directly obtain a complete absorption spectrum including the absorption center and its two wings by comparing the transmitted laser intensity with the reference laser intensity, and derives the parameters by fitting the complete absorption spectrum. In addition, the Doppler frequency shift can be directly extracted by comparing with other absorption spectra in static or laser paths with different laser directions. However, the accuracy of the absorption spectrum is sensitive to the reference laser intensity, which is usually fitted from the transmitted laser signal. In the actual measurement environment of high temperature and high-speed airflow, non-absorption losses caused by beam steering effects or particle scattering can cause fluctuations in the transmission signal, which affects the fitting accuracy of the reference laser intensity and limits the accuracy of the absorption spectrum measurement.

[0005] Unlike the DAS method, the WMS method modulates the laser at high frequency and orthogonally demodulates the transmitted WMS signal to obtain a high-precision harmonic signal of the absorption spectrum. The normalized 2f / 1f harmonic based on the WMS method can effectively avoid background noise and fluctuations in laser signal intensity, and is widely used in gas concentration and temperature detection. However, the velocity measurement based on the Doppler effect relies on the measurement of laser frequency offset, and the harmonics extracted from the transmitted WMS signal using the time-based phase-locked amplification technology lack clear corresponding frequency coordinate information. Therefore, it is impossible to directly use the extracted Doppler frequency shift information of the harmonic signal to obtain the velocity. In some studies, it is usually considered to use the peak point of the harmonic to correspond to the central wavelength of the absorption spectrum. However, this assumption usually needs to meet complex restrictions, such as the phase difference between wavelength modulation and intensity modulation is π. Therefore, although the WMS method can obtain high-precision harmonic signals, extracting the Doppler frequency shift of the laser from the harmonic signal still requires a complex calibration process. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide an absorption spectrum gas velocity measurement method for constructing and matching absorption peak image features.

[0007] To achieve the above object, the present invention adopts the following technical solution:

[0008] A laser absorption spectroscopy gas velocity measurement method, comprising:

[0009] Step S1, performing multiple phase shift downsampling on the detected wavelength modulated laser signal, reconstructing the multiple phase shift sequences, and obtaining a two-dimensional grayscale image matrix containing a direct absorption spectrum profile;

[0010] Step S2, obtaining an image template representing the absorption peak position according to the two-dimensional grayscale image matrix;

[0011] Step S3, determining the offset of the absorption peak position in two directions according to the image template of the absorption peak position;

[0012] Step S4: Calculate the airflow velocity using the absorption peak offsets in the two detection directions.

[0013] Preferably, in step S2, a sharpening process is performed on the two-dimensional grayscale image matrix extracted from the laser signal with zero flow field velocity to obtain an image template representing the absorption peak position.

[0014] Preferably, in step S3, based on the image template of the absorption peak position, the position of the matching image template in the two-dimensional grayscale image obtained from the laser detection signals in two directions in the flow field to be measured is used to determine the offset of the absorption peak position in two directions.

[0015] The present invention reconstructs a two-dimensional image matrix containing a direct absorption spectrum profile using a phase shift sequence obtained by performing multiple phase shift downsampling on a WMS signal, extracts the Doppler frequency shift of the absorption spectrum from the sharpening features of the direct absorption spectrum profile, and uses a template image to perform image feature matching to achieve airflow velocity measurement; the present invention improves the accuracy of solving the absorption spectrum frequency shift at different optical depths (i.e., the size of the absorption rate), thereby improving the velocity measurement accuracy in a complex flow field with changing working conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0017] Figure 1 A flow chart of a method for measuring gas velocity using absorption spectrum by constructing and matching absorption peak image features according to an embodiment of the present invention;

[0018] Figure 2 It is the optical path diagram of laser absorption spectrum;

[0019] Figure 3 This is a schematic diagram of the cross optical path test;

[0020] Figure 4 It is a schematic diagram of a discretized wavelength modulation signal;

[0021] Figure 5 Downsampling and rearrangement matrix plane diagram for synthesis;

[0022] Figure 6 Downsampled and rearranged matrix stereogram for synthesis;

[0023] Figure 7 This is a schematic diagram of an image after being sharpened using filtering;

[0024] Figure 8 The speed diagram is obtained by using image feature matching. DETAILED DESCRIPTION

[0025] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0026] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0027] Embodiment 1:

[0028] like Figure 1 As shown, an embodiment of the present invention provides an absorption spectrum gas velocity measurement method for constructing and matching absorption peak image features, including:

[0029] Step S1, performing multiple phase shift downsampling on the detected wavelength modulated laser signal, reconstructing the multiple phase shift sequences, and obtaining a two-dimensional grayscale image matrix containing a direct absorption spectrum profile;

[0030] Step S2, sharpening the two-dimensional grayscale image matrix extracted from the laser signal with zero flow field velocity to obtain an image template representing the absorption peak position;

[0031] Step S3, according to the image template of the absorption peak position, the position of the matching image template in the two-dimensional grayscale image obtained from the laser detection signals in two directions in the flow field to be measured is determined to determine the offset of the absorption peak position in the two directions;

[0032] Step S4: Calculate the airflow velocity using the absorption peak offsets in the two detection directions.

[0033] As an implementation of an embodiment of the present invention, in step S1, WMS is a typical TDLAS method with excellent anti-noise performance. The driving current of the emitted WMS laser consists of two components, the low-frequency component comes from the sawtooth wave of the low-frequency scanning, and the high-frequency component comes from the sine wave scanning. The emitted laser wave number can be expressed as:

[0034]

[0035] Where v0 is the center frequency of the absorption spectrum. s (t) is a function of the laser frequency slow scan as a sawtooth profile, v m and α m is the modulated laser in mf H The modulation depth and phase shift at . The emitted laser intensity I0 is expressed as:

[0036]

[0037] Among them, I s (t) is the laser intensity driven by a low-frequency sawtooth wave, b n and β nis the intensity amplitude and phase shift of the laser modulated in a high frequency sine wave. When the emitted laser passes through the target gas, the emitted WMS laser signal is subsequently detected by a photodiode detector. The intensity of the transmitted WMS laser signal is then recorded by DAQ (data acquisition) with a sampling rate of f s .

[0038] When the laser signal is transmitted through the gas to be measured, Figure 2 As shown in the figure, the change of gas absorptivity with wavelength due to absorption effect can be expressed as the detected transmitted laser intensity I t The ratio of the emitted laser intensity I0 can also be expressed as:

[0039]

[0040] Where l[m] is the laser absorption path length, P[atm] is the pressure, T(x)[K] and X abs (x) are the gas temperature and concentration along the gas absorption path, Δv u Due to the air flow velocity The laser Doppler effect caused by can be expressed as:

[0041]

[0042] Where c represents the speed of light and θ represents the propagation direction of the detection laser. and flow velocity The angle of direction, such as Figure 3 As shown, two directions of laser light paths are usually used to test simultaneously to eliminate the common mode, and the flow field velocity value can be expressed as u:

[0043]

[0044] Among them, θ1 and θ2 are the angles between the two laser propagation directions and the airflow velocity direction. and Represents the laser Doppler effect in two directions. Therefore, for the detected transmitted laser intensity I in a single scanning cycle t Digitally represented, it can be expressed as

[0045]

[0046] Where N is the number of sampling points in a single scanning cycle. Let the low frequency value be f L , then N can be expressed as:

[0047] N = f S / f L (7)

[0048] Downsampling is a signal processing technique that involves selecting a certain number of values ​​from a continuous sequence of samples to form a new sequence with a reduced sampling frequency. Figure 4 As shown in the original sequence of the digitally transmitted WMS signal, let Q be the total number of cycles that the high-frequency sine wave goes through during the low-frequency sawtooth wave scanning process, and let P be the total number of sampling points in a single sine wave cycle. Since the time intervals between two adjacent sampling points are equal, the phase difference between the two adjacent sampling points is defined as It is expressed as:

[0049]

[0050] Downsampling still needs to follow the sampling theorem, otherwise it may cause signal distortion and aliasing. Define M as the downsampling interval:

[0051]

[0052] This means that the sampling frequency of the new downsampled sequence is reduced to f s / M. At the same time, let each sampling point (number P) in the first sine cycle be an independent starting point of each down-sampling sequence.

[0053] These down-sampling sequences are named polyphase sequences. i represents the set of the i-th multiphase sequence, denoted as:

[0054] I i =[I t (k)|k=jM+i,0 <j≤Q,j∈Z] (10)

[0055] Substituting formula (7) into formula (6), we get:

[0056]

[0057] in, and is an invariant in the set of i-th multiphase sequences, since i is an invariant. According to this, I i The waveform is a sawtooth wave scan waveform, such as Figure 5 As shown, it can also be viewed as a transmitted DAS profile signal.

[0058] Further, these polyphase sequences are rearranged to create a Q×P matrix, named DR matrix (downsampling and rearrangement matrix), denoted as:

[0059]

[0060] In the different rows of I, these are different multiphase sequences. When the gas velocity is 0, the position of the absorption peak changes with The function can be expressed as:

[0061]

[0062] As an implementation of an embodiment of the present invention, in step S2, the intensity change of the rearranged down-sampled signal in the DR matrix expresses the position of the absorption peak caused by the absorption effect of the gas molecules. The position of the absorption peak can be determined by the feature matching method. Figure 6 As shown in the figure, the absorption peak feature is masked by the intensity variation of the sawtooth laser scanning. Sharpening this feature helps to accurately extract the position of the absorption spectrum in a noisy environment. Removing the intensity variation caused by the laser scanning is beneficial to sharpen this feature and further improve the ability to accurately extract the position of the absorption spectrum in a high noise environment and under small absorption conditions.

[0063] Let I G represents the sharpened DR matrix, and the sharpening kernel is G. According to the downsampling rule, for any random point in the i-th row and j-th column in the DR matrix, its adjacent points in the original digital transmission WMS signal are rearranged in the (i+1)th row and (j-1)th column and the (i-1)th row and (j+1)th column. Therefore, the sharpening operation is expressed as:

[0064]

[0065] Where “*” indicates a convolution operation. N Represents I normalized in the range of -1 to 1 G The above operation makes the characteristics of the absorption spectrum significantly enhanced, and there is a transition from the maximum value to the minimum value near the absorption peak.

[0066] like Figure 7 As shown, the contrast of the absorption peak feature after sharpening is more obvious. Then, by calculating the pixel position change of the absorption spectrum feature in the image, the frequency shift of the absorption spectrum caused by the Doppler effect can be calculated, thereby confirming the airflow velocity.

[0067] Let I N|u＝0 It represents the sharpened DR matrix of the transmitted laser signal when the airflow velocity is 0. Then, the matching template I about the absorption feature is extracted from the normalized sharpened DR matrix K , expressed as:

[0068]

[0069] Where, “°” represents the Hadamard product of two matrices. In formula (12), I K is set to a (S2-S1)×Q matrix. The absorption features exist in rows S1 to S2. In addition, to simplify the solution, IK Set to a matrix with Q columns to avoid eigenmatching in the column direction.

[0070] As an implementation of the embodiment of the present invention, in step S3,

[0071] Let I N|u represents the sharpened DR matrix of the transmitted laser signal when the flow rate is u. According to formula (13), the position of the absorption peak can be expressed as:

[0072]

[0073] Among them, j u and j0 represent the positions of the absorption peaks when the gas velocity is u and 0, respectively. Usually, the wavelength scanning function of the sawtooth wave can be adjusted to a first-order linear function, which can be further obtained:

[0074]

[0075] Among them, Δj u It represents the position shift of the absorption peak characteristic caused by the Doppler frequency shift when the flow velocity is u. α is v s The first-order coefficient of .

[0076] Furthermore, if Figure 8 As shown, the position of the absorption spectrum feature is calculated by using the template image for image feature matching, assuming ρ u Indicates I K and I N|u The normalized correlation coefficient between is expressed as:

[0077]

[0078] in, Indicates I N|u The average intensity of the local part is calculated as:

[0079]

[0080] Let ρ u is u The position of the maximum value in (p):

[0081]

[0082] Among them, ρ u Indicates the change in the position of the absorption peak; in determining ρ u When we select a value close to ρ u To fit the extreme points, we can obtain non-integer ρ u To improve the measurement resolution.

[0083] As an implementation method of the present invention, in step S4, by combining formula (17) and disclosure (5), the gas velocity can be determined as:

[0084]

[0085] in, and is when u takes the value of u1 and u2 u The value of .

[0086] The present invention improves the accuracy of gas velocity measurement based on TDLAS, especially for velocity measurement in high-temperature and high-speed environments such as engine exhaust nozzles. The multiphase sequence extracted from the transmitted WMS signal by the downsampling strategy is rearranged into a grayscale image corresponding to the laser frequency coordinate, that is, the 1-D WMS signal is converted into a 2-D grayscale image in which each line is a DAS profile with laser frequency coordinates. An image filtering method is used to sharpen the absorption spectrum features in the two-dimensional grayscale image, instead of the reference laser intensity fitting in the traditional DAS method. It can avoid non-absorption losses that affect the reference laser intensity fitting accuracy and limit the absorption spectrum measurement accuracy. An image feature matching method is used to calculate the Doppler shift to derive the velocity, and the intensity and two-dimensional distribution of the absorption spectrum features are used to improve the matching accuracy within a wide absorption range. The proposed image processing method has an important role in promoting advanced TDLAS parameter solutions, and also provides the possibility of using GPU devices that are good at parallel image computing to quickly solve TDLAS parameters online in the future.

[0087] The embodiments described above are only descriptions of the preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A laser absorption spectroscopy gas velocity measurement method, characterized in that: include: Step S1, performing multiple phase shift downsampling on the detected wavelength modulated laser signal, reconstructing the multiple phase shift sequences, and obtaining a two-dimensional grayscale image matrix containing a direct absorption spectrum profile; Step S2, obtaining an image template representing the absorption peak position according to the two-dimensional grayscale image matrix; Step S3, determining the offset of the absorption peak position in two directions according to the image template of the absorption peak position; Step S4: Calculate the airflow velocity using the absorption peak offsets in the two detection directions.

2. The laser absorption spectroscopy gas velocity measurement method according to claim 1, characterized in that: In step S2, a two-dimensional grayscale image matrix extracted from a laser signal with a flow field velocity of zero is sharpened to obtain an image template representing the absorption peak position.

3. The laser absorption spectroscopy gas velocity measurement method according to claim 2, characterized in that: In step S3, according to the image template of the absorption peak position, the position of the image template is matched in the two-dimensional grayscale image obtained from the laser detection signals in two directions in the flow field to be measured, and the offset of the absorption peak position in two directions is determined.

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