CN115979997B - A phase-locked amplification method based on linear convolution in TDLAS gas detection - Google Patents
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Abstract
本发明公开了一种TDLAS气体检测中基于线性卷积的锁相放大方法，该方法将气体分子吸收光谱信号与正/余弦参考信号进行线性卷积，利用线性卷积中积分器的低通滤波特性可将卷积序列中的高频项近似为0，最后经过平方和即可获取多次谐波信号。本发明通过较为简便的方法实现了锁相放大功能，相比传统锁相放大方法，计算速度更快，占用资源更少。
The present invention discloses a phase-locked amplification method based on linear convolution in TDLAS gas detection. The method linearly convolves the gas molecule absorption spectrum signal with the sine/cosine reference signal, and uses the low-pass filtering characteristics of the integrator in the linear convolution to approximate the high-frequency terms in the convolution sequence to 0, and finally obtains multiple harmonic signals through square sum. The present invention realizes the phase-locked amplification function through a relatively simple method, and has faster calculation speed and less resource occupation than the traditional phase-locked amplification method.
Description
Technical Field
The invention relates to the technical field of gas detection, in particular to a phase-locked amplification method based on linear convolution in TDLAS gas detection.
Background
With the rapid development of industry, new requirements are continuously put forward in the field of gas detection, such as industrial gas detection, vehicle tail gas detection, medicine bottle residual oxygen detection and the like.
At present, the tunable diode laser absorption spectrum (Tunable Diode Laser Absorption Spectroscopy, TDLAS) technology is an ideal high-sensitivity rapid trace gas detection method and is widely applied to various gas detection fields. The wavelength modulation spectrum (WAVELENGTH MODULATION SPECTROSCOPY, WMS) technology is often used in combination with the TDLAS technology, so that the detection system has an extremely high noise suppression capability. In a trace gas detection system based on the TDLAS-WMS technology, the phase-locked amplification technology plays a crucial role. Specifically, by using the lock-in amplification technology, multiple harmonics can be demodulated from the absorption spectrum signal by setting a reference signal, which becomes an important basis for gas analysis. Therefore, lock-in amplification technology plays a very important role in TDLAS-WMS based gas detection systems.
The existing phase-locked amplifying method mainly adopts a quadrature phase-locked method, but the method needs two-way multiplication and two-way low-pass filters, and when hardware resources are limited, the problems of performance degradation or filter order difficulty are solved.
Disclosure of Invention
Aiming at the problems in the related art, the invention provides a phase-locked amplification method based on linear convolution in TDLAS gas detection so as to realize harmonic extraction of gas absorption spectrum signals, and compared with a quadrature phase-locked amplification method, the phase-locked amplification method has less calculation resource occupation.
For this purpose, the invention adopts the following specific technical scheme:
A phase-locked amplifying method based on linear convolution in TDLAS gas detection comprises the following steps of 1, carrying out linear convolution on a gas molecule absorption spectrum signal and a sine/cosine reference signal, 2, approximating a high-frequency item in a convolution sequence to 0 by utilizing a low-pass filter characteristic of an integrator in the linear convolution, and 3, obtaining a plurality of harmonic signals through square sum.
In a possible design, step 1 further includes performing signal acquisition on the absorption spectrum signal at a sampling rate f s, and performing fourier expansion on the expression and representing the expression as a discrete sequence, where the following formula is shown:
Wherein I is the original light intensity, v 0 is the scanning wavelength amplitude, v m is the modulating wavelength amplitude, ω is the high-frequency cosine signal frequency, H r is the Fourier expansion series, and phi r is the Fourier expansion r-stage phase.
In one possible design, in step 1, the set sine/cosine reference signals are respectively: and carrying out linear convolution on the absorption spectrum signal discrete sequence and the reference signal discrete sequence to obtain two paths of convolution sequences X (n) and Y (n).
In one possible design, in step 2, when the high frequency sinusoidal signal frequency in the modulated signal is high enough (i.e., ω is large enough) and the sampling rate is sufficient, the following is taken as a divisor:
in one possible design, in step 2, the convolution sequence X (n) and the convolution sequence Y (n) may be approximated by the following formula:
in one possible design, in step 3, the sum of squares is applied to the two convolution sequences X (n) and Y (n) to obtain a discrete sequence Z (n) expressed as:
in one possible design, in step 3, the discrete sequence Z (n) is the k-th harmonic.
The invention also provides a gas detection device based on the TDLAS-WMS technology, which comprises a memory, a control processor and a computer program which is stored in the memory and can run on the control processor, wherein the control processor executes the program to realize the phase-locked amplifying method.
The invention also provides a gas detection system which comprises the gas detection device based on the TDLAS-WMS technology.
The present invention also provides a computer-readable storage medium storing computer-executable instructions for causing a computer to perform the aforementioned lock-in amplification method.
Compared with the prior art, the invention has the beneficial effects that:
The method has the beneficial effects that the method is rapid and accurate, can realize phase-locked demodulation of the absorption spectrum signals, and provides technical support for analyzing trace gas. Compared with a phase-locked amplifying method based on quadrature demodulation, the method does not need to be processed by a low-pass filter, and is faster in calculation speed and less in occupied resources.
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In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings that are needed in the description of the embodiments will be briefly described below, it being obvious that the drawings in the following description are only some embodiments of the present application, and that other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
FIG. 1 shows a flow chart of an embodiment of a phase-locked amplification method based on linear convolution in TDLAS gas detection according to the present invention;
Fig. 2 shows the first to fourth harmonics demodulated in the embodiment of the phase-locked amplifying method based on linear convolution in TDLAS gas detection according to the present invention.
Detailed Description
For the purpose of further illustrating the various embodiments, the present invention provides the accompanying drawings, which are a part of the disclosure of the present invention, and which are mainly used to illustrate the embodiments and, together with the description, serve to explain the principles of the embodiments, and with reference to these descriptions, one skilled in the art will recognize other possible implementations and advantages of the present invention, wherein elements are not drawn to scale, and like reference numerals are generally used to designate like elements.
The phase-locked amplifying method based on linear convolution for the TDLAS gas detection system provided by the embodiment comprises the following steps:
A set of gas detection system based on the TDLAS-WMS technology is built, a superposition signal of a low-frequency slope signal and a high-frequency cosine signal is used as a current modulation signal of a laser, and under the modulation mode, the laser light intensity output by the laser can be expressed as follows:
The laser is transmitted in free space and is absorbed by the gas to be detected, a photoelectric detector is arranged to convert an optical signal into an electric signal, and the spectral signal expression after gas absorption is as follows:
The signals are collected, the sampling rate is set to fs, and an N-point absorption spectrum signal discrete sequence I t (N) is obtained, and the expression is as follows:
setting sine/cosine k frequency-doubled reference signal sin (kωt)/cos (kωt), at a sampling rate fs, the discrete sequence of two reference signals can be expressed as:
the reference signal discrete sequence takes only one period, so the length of the discrete sequence is
Discrete sequence of absorption spectrum signal I t (n) and discrete sequence of sine reference signalPerforming linear convolution to obtain a convolution sequence X (n), wherein the expression is as follows:
To analyze the result of the above-described discrete sequence expansion, the absorbance spectrum signal is now fourier-expanded, and the following expression can be obtained:
representing the modified fourier expansion as a discrete sequence yields the following expression:
the convolution sequence X (n) can be expressed as:
It is known that an integrator can be simply considered as a low-pass filter, and when the frequency of the high-frequency sinusoidal signal in the modulated signal is high enough (i.e., ω is large enough) and the sampling rate is large enough, it can be divided into the following terms:
The convolution sequence X (n) may be approximated by:
discrete sequence of absorption spectrum signal I t (n) and discrete sequence of cosine reference signal Performing linear convolution to obtain a convolution sequence Y (n), wherein the expression is as follows:
With the convolution sequence X (n), fourier expansion is carried out on the absorption spectrum signal sequence items in the convolution sequence Y (n), and the expression of the convolution sequence Y (n) can be obtained as follows:
With the convolution sequence X (n), the convolution sequence Y (n) can also be approximated by the following expression due to the low-pass filtering function of the integrator:
Further, in order to finally realize the phase-locked amplifying function, the square sum of the two convolution sequences X (n) and Y (n) is performed to obtain a discrete sequence Z (n), and the expression is as follows:
Referring to the approximate expression of the convolution sequence X (n) and the convolution sequence Y (n), the discrete sequence Z (n) may be approximated as follows:
because the filter function of the integrator is limited, partial high-frequency noise still can exist in the obtained discrete signal, and the discrete sequence is k times of harmonic waves.
The invention also provides a gas detection device based on the TDLAS-WMS technology, which comprises a memory, a control processor and a computer program which is stored in the memory and can run on the control processor, wherein the control processor executes the program to realize the phase-locked amplifying method.
The invention also provides a gas detection system which comprises the gas detection device based on the TDLAS-WMS technology. The present invention also provides a computer-readable storage medium storing computer-executable instructions for causing a computer to perform the aforementioned lock-in 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, nonvolatile 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 memory, hard Disk Drive (HDD), solid State Disk (SSD), card memory (such as a multimedia card, secure Digital (SD) card, or ultra-digital (XD) card), magnetic tape, floppy disk, magneto-optical data storage device, hard disk, solid state disk, and any other device configured to store and provide and any associated data, data files and data structures to and to a computer program or processor or to cause the computer to execute the computer program or data storage.
In one example, the computer program and any associated data, data files, and data structures are distributed across networked computer systems such that the computer program and any associated data, data files, and data structures are stored, accessed, and executed in a distributed manner by one or more processors or computers.
The method has the beneficial effects that the method is rapid and accurate, can realize phase-locked demodulation of the absorption spectrum signals, and provides technical support for analyzing trace gas. Compared with a phase-locked amplifying method based on quadrature demodulation, the method does not need to be processed by a low-pass filter, and is faster in calculation speed and less in occupied resources.
Specifically, steam is selected as the gas to be measured, and the operation flow is shown in fig. 1.
Firstly, an experimental platform is built, and a steam laser, a laser controller ITC4001, a laser detector PDA10DTEC, a data acquisition card USB-6361 and a PC are connected according to requirements.
Setting 185mv-265mv and 10Hz sawtooth wave plus 20mv 6kHz sine wave as control current signals of a laser, stabilizing and outputting laser with a wave band 1392.45-1392.55 nm, absorbing the emitted laser by water vapor in air, receiving the laser by a laser detector PDA10DTEC, collecting the received gas absorption spectrum signals by a data collecting card, setting the sampling rate to 300K, and transmitting the collected signals to a PC for further data processing.
The sine/cosine reference signals with 1/2/3/4 frequency multiplication (6 kHz/12kHz/18kHz/24 kHz) are respectively taken, the discrete sequence of each reference signal under 300K sampling rate is obtained, and only one period is taken.
The acquired absorption spectrum signal discrete sequence is respectively linearly convolved with each frequency-doubled sine/cosine reference signal discrete sequence to respectively obtain two paths of signals corresponding to different orders X1(n)、Y1(n)/X2(n)、Y2(n)/X3(n)、Y3(n)/X4(n)、Y4(n).
And respectively performing square sum operation on the two paths of signals to obtain a discrete sequence Z 1(n)、Z2(n)、Z3(n)、Z4 (n) which is 1/2/3/4 th harmonic.
While, for purposes of simplicity of explanation, the methodologies are shown and described as a series of acts, it is to be understood and appreciated that the methodologies are not limited by the order of acts, as some acts may, in accordance with one or more embodiments, occur in different orders and/or concurrently with other acts from that shown and described herein or not shown and described herein, as would be understood and appreciated by those skilled in the art.
The foregoing description of the preferred embodiments of the invention is not intended to be limiting, but rather is intended to cover all modifications, equivalents, alternatives, and improvements that fall within the spirit and scope of the invention.
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