GB2608554A - Multi-frequency modulation method for gas concentration measurement, gas concentration measurement method and system, and apparatus - Google Patents
Multi-frequency modulation method for gas concentration measurement, gas concentration measurement method and system, and apparatus Download PDFInfo
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- 238000000041 tunable diode laser absorption spectroscopy Methods 0.000 claims description 5
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- 239000007789 gas Substances 0.000 description 48
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- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
- G01N21/39—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using tunable lasers
- G01N2021/396—Type of laser source
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Abstract
A multi-frequency modulation method for gas concentration measurement, a gas concentration measurement method and system, and an apparatus. The multi-frequency modulation method comprises: step S1, adjusting the amplitude of a basic modulation signal, so that the abscissa of a peak point in a secondary harmonic absorption curve output by a multi-frequency modulation gas concentration measurement system is the same as the abscissa of a minimum point enveloped by background interference fringes; and step S2, respectively adjusting the amplitude and frequency of a jitter modulation signal on the basis of an amplitude adjusting interval and a frequency adjusting interval of the jitter modulation signal. The jitter modulation signal is added to an original single-frequency modulation system to constitute a multi-frequency modulation system, and by adjusting the amplitude of the basic modulation signal and the amplitude and frequency of the jitter modulation signal, background interference fringe noises can be effectively suppressed, thereby facilitating improving the signal-noise ratio and the gas concentration measurement accuracy of the system.
Description
GAS CONCENTRATION MEASUREMENT METHOD AND APPARATUS BASED ON:NIULTI-FREQUENCY MODULATION, AND GAS CONCENTRATION MEASUREMENT METHOD AND SYSTEM
TECHNICAL FIELD
[00011 The present disclosure belongs to the technical field of gas measurement; and specifically relates to a gas concentration measurement method and apparatus based on multi-frequency modulation, a gas concentration measurement method and system.
BACKGROUND
100021 Based on tunable laser light, tunable diode laser absorption spectroscopy (IDEAS) is a new spectroscopic detection technology developed in recent years. 'IDLAS technology uses a non-contact measurement method, which can accurately select and analyze a sample to be measured, and has the advantages of high detection sensitivity, high resolution, and good real-time performance. Therefore, 'IDEAS technology has become one of the key means of online gas measurement.
[0003] Multiple reflections of laser light on a surface of an optical element may cause interference fringes, which are manifested as periodic low-frequency noise mixed in a harmonic signal, and exert a great impact on the measurement of a gas concentration characteristic value interference fringes may be influenced by various factors such as a drive current" laser temperature, and etalon length. in order to effectively suppress the interference fringes, researchers have proposed some feasible methods. Guo Xinqian et al proposed a method of multiple denoising based on empirical mode decomposition to reduce interferometric noise in multi-optical-range absorption spectra. 13.5 Bomse et al. proposed an adaptive singular value decomposition algorithm, Alan Fried et al introduced a semiconductor laser using a pressure modulation technology, which can minimize interferometric fringes generated in a multi-pass absorption cell. In addition, other scholars have proposed methods such as balanced gas measurement, Fourier analysis; and digital signal processing. These methods can help effectively suppress interference fringes. However, these methods may increase the complexity of the system software algorithm and hardware structure, affect real-time system performance, and show poor adaptability to measurement objects.
SUADIARY
[00041 An objective of the present disclosure is to provide a gas concentration measurement method and apparatus based on multi-frequency modulation, a gas concentration measurement method and system. From the perspective of a tunable diode laser absorption spectroscopy (IDLAS)-wavelength modulation spectroscopy (AVMS) system modulation signal, the method adds a dither modulation signal on the basis of an original modulation signal without increasing the system complexity, and there is no need to add a new complex noise suppression algorithm. The method can effectively suppress background interference fringe noise, and helps improve the signal-to-noise ratio and the gas concentration measurement accuracy of the system.
[0005] According to an aspect, the present disclosure provides a. gas concentration measurement method based on multi-frequency modulation, including: 100061 step Si: adjusting an amplitude of a. basic modulation signal such that a horizontal coordinate of a pea.k point of a second harmonic absorption curve outputted by a multi-frequency modulation gas concentration measurement system is the same as a horizontal coordinate of an envelope minimum point of background interference fringes, [0007] where a dither modulation signal is added to a single-frequency modulation gas concentration measurement system based on tunable diode laser absorption spectroscopy (MLA S).-wavelength modulation spectroscopy ( \VMS), to obtain the multi-frequency modulation gas concentration measurement system; a second harmonic signal outputted by the multi-frequency modulation gas concentration measurement system is formed by a second harmonic absorption signal superimposed with the background interference fringes; and an initial frequency value and an initial amplitude value of the added dither modulation signal are preset values; and [0008] step 52: adjusting an amplitude and a frequency of the dither modulation signal based on an amplitude adjustment interval and a frequency adjustment interval of the dither modulation signal 100099 To reduce the impact of the background interference fringes, the present disclosure adds a dither modulation signal to the original modulation system to constitute a multi-frequency modulation system. Moreover, researches show that the background interference fringes are influenced by an amplitude mf of the basic modulation signal and an amplitude m2 of the dither modulation signal. In order to suppress the background interference fringe noise, the background interference fringes can be suppressed by adjusting the amplitude m1 of the basic modulation signal and the amplitude m2 of the dither modulation signal. The amplitude mu of the basic modulation signal not only affects the noise level of the background interference fringes but also affects the second harmonic absorption peak signal. Therefore, mi is first adjusted such that the horizontal coordinate of the peak point of the second harmonic waves is the same as the horizontal coordinate of the envelope minimum point of the background interference fringes. Such an adjustment of m; can not only achieve a large value of the absorption signal but also reduce the background interference frir io se to some extent, the amplitude 1722 and frequency at? of the dither modulation signal are adjusted. During adjustment of the amplitude m2 and frequency 4702 of the dither modulation signal, any value within the amplitude adjustment interval and any value within the frequency adjustment interval can be taken, or values can be selected according to the following method described in the present disclosure. Preferably, the values are selected according to the following method, to suppress the background interference hinge noise more effectively.
[0010] Further preferably, a theoretic:al optinal amplitude value of the dither modulation signal is at least within the amplitude adjustment nterval, and is obtained when a zero-order.Bessel function term reaches a first zero point.
10011.1 Further preferably, the theoretical optimal amplitude value of the dither modulation signal is as follows: [0012] Al, = 0.383/ 2/J [0013] where -A44is the theoretical optimal amplitude value of the 'ther modulation signal, 1 is an etalon length, and is a laser frequency-current tuning factor. When the zero-order Bessel function term reaches the first zero point, A/12 = 0-383 is approximately satisfied, while = 21m1e. Therefore, the above calculation formula for the theoretical optimal amplitude value AC of the amplitude m2 of the dither modulation signal is obtained.
[0014] Further preferably, the amplitude adjustment interval is: [5, juk, and kft d is the theoretical optimal amplitude value of the dither modulation signal.
[0015] Further preferably, the process of adjusting an amplitude and a frequency of the dither modulation signal based on an amplitude adjustment interval and a frequency adjustment interval of the dither modulation signal is as follows: 10016] adjusting the amplitude of the dither modulation signal at an equal step of /Mc within the amplitude adjustment interval, collecting N peak values of the second harmonic absorption curve after each amplitude adjustment, then comparing noise levels of the peak values of the second harmonic absorption curve under different amplitudes, and finally selecting an amplitude corresponding to a lowest noise level as the amplitude of the dither modulation signal, where Nis a positive integer; and [0017] adjusting the frequency of the dither modulation signal at an equal step of 4w within the frequency adjustment interval, collecting AT peak values of the second harmonic absorption curve after each frequency adjustment, then comparing noise levels of the peak values of the second harmonic absorption curve under different frequencies and finally selecting a frequency corresponding to a lowest noise level as the frequency of the dither modulation siDtal.
[0018] A parameter for measuring the noise level may be a standard deviation of the second harmonic peak values, an Allan variance, or another parameter, whir.h is not specifically limited in the present disclosure. Moreover, to exclude the interference of environmental factors and the like, the above processes can be repeated K1 and K2 times respectively.
100191 Further preferably, the frequency adjustment interval * [0.2cor, 0.4011, where cur is a frequency of the basic modulation signal.
[00201 The frequency cur of the dither modulation signal does not have a theoretical optimal value. When 02 is set to an integer multiple of co', such as arl, 2w,i, or Rol, the second harmonic signal is generally distorted; when to2-(0.2(of, the background interference fringe noise will increase. Considering the burden of the system hardware, the value of air should not be excessively large. Therefore, the value of is generally 0.2oil to ()Awl.
100211 Further preferably, in step S2, the amplitude of the dither modulation signal is adjusted first, and then the frequency of the dither modulation signal is adjusted.
[0022] According to another aspect, the present disclosure provides a gas concentration measurement method based on multi-frequency modulation, where when a concentration of another gas object is measured by a gas concentration measurement system with an amplitude of a basic modulation signal and an amplitude and a frequency of a dither modulation signal determined by using the foregoing multi-frequency modulation method, the multi-frequency modulation method includes: [00231 building a gas concentration measurement system modulated according to the multi-frequency modulation method during measurement of a previous gas, to measure a concentration of another gas object; and 1.0024] adjusting the amplitude of the basic modulation signal such that a horizontal coordinate of a peak point of a second harmonic absorption curve outputted by the gas concentration measurement system is the same as a horizontal coordinate of an envelope minimum point of current background interference fringes.
100251 In addition, the present disclosure further provides a gas concentration measurement method, including: [0026] adjusting an amplitude of a basic modulation signal and an amplitude and a frequency of a dither modulation signal by using the above multi-frequency modulation method; and [00271 measuring a ga.s concentration based on a modulated ga.s concentration measurement system.
[0028] Finally, the present disclosure further provides a gas concentration measurement system, including: a lock-in amplifier, a laser driver; a temperature control module, a laser, a glass bottle, and a photoelectric detector, where the laser driver and the photoelectric detector are both connected to the lock-in amplifier, the temperature control module is built in the laser driver, the laser driver and the temperature control module are connected to the laser, the glass bottle is disposed at a laser transmitting end of the laser and is arranged between the laser and the photoelectric detector, and the glass bottle contains a gas whose concentration is to be measured; 100291 the lock-in amplifier outputs a driving current signal to the laser driver, where the driving current signal is formed by low-frequency sawtooth waves, a basic modulation signal, and a dither modulation signal, and an amplitude of the basic modulation signal and an amplitude and a frequency of the dither modulation signal are determined according to the above multi-frequency modulation method; [0030] the laser driver generates a control current after receiving the driving current signal from the lock-in amplifier, and jointly controls, with the temperature control module, the laser to emit modulated light; and 1.0031] the modulated light emitted by the laser is received by the photoelectric detector after passing -through the glass bottle, the photoelectric detector converts an optical signal into an absorption electrical signal and transmits the absorption electrical signal to the lock-in amplifier for demodulation, and the lock-in amplifier outputs a second harmonic signal after demodulation. [0032] Further preferably, the gas concentration measurement system further includes a digital oscilloscope or a digital processor connected to the lock-in amplifier. The digital oscilloscope is configured to observe the second harmonic signal, and the digital processor may be configured to process data.
[00331 The digital processor is connected to the lock-in amplifier, and the lock-in amplifier outputs the second harmonic signal to the digital processor; and 1.0034] the digital processor executes the following instructions: [0035] identifying whether a horizontal coordinate of a peak point of a second harmonic absorption curve is equal to a horizontal coordinate of an envelope minimum point of background interference fringes according to the second harmonic signal; if yes, feeding back a signal to the lock-in amplifier, and keeping the current amplitude of the basic modulation signal unchanged; otherwise, feeding back an amplitude adjustment instruction for the basic modulation signal to the lock-in amplifier, such that the lock-in amplifier performs adjustment; and [0036] feeding back an amplitude adjustment instruction and a frequency adjustment instruction for the dither modulation signal to the lock-in amplifier based on an amplitude adjustment interval and a frequency adjustment interval of the dither modulation signal, such that the lock-in amplifier performs adjustment.
[0037] According; to another aspect, the present disclosure provides a gas concentration measurement apparatus based on multi-frequency modulation, including a lock-in amplifier, a digital processor, and a memory that are connected to one another; where 100381 the lock-in amplifier outputs a driving current signal to a laser driver of a gas concentration measurement system, receives an absorption electrical signal from a photoelectric detector in the gas concentration measurement system, and outputs a second harmonic signal to the digital processor after demodulation, where the driving current signal is formed by low-frequency sa,wtooth waves, a basic modulation signal, and a dither modulation signal; and [0039] the digital processor is connected to the memory storing instructions, and the digital processor executes the instructions stored in the memory to implement the following operations: [0040] identifying whether a horizontal coordinate of a peak point of a second harmonic absorption curve is equal to a horizontal coordinate of an envelope minimum point of background interference fringes according to the second harmonic signal; if es, feeding back a signal to the lock-in amplifier, and keeping the current amplitude of the basic modulation signal unchanged; otherwise, feeding back an amplitude adjustment instruction for the basic modulation signal to the lock-in amplifier, such that the lock-in amplifier performs adjustment; and [0041] feeding back an amplitude adjustment instruction and a frequency adjustment instruction for the dither modulation signal to the lock-in amplifier based on an amplitude adjustment interval and a frequency adjustment interval of the dither modulation signal, such that the lock-in amplifier performs adjustment.
[0042] Beneficial effects 100431 On the one hand, the multi-frequency modulation method provided by the present disclosure adds a dither modulation signal to the original single-frequency modulation system to constitute a multi-frequency modulation system. In this way, no additional hardware elements or noise suppression algorithm is added, thereby avoiding increasing the system complexity due to the use of a complex data processing manner and the addition of system elements. After the introduction of the dither modulation signal, the amplitude of the basic modulation signal and the amplitude and frequency of the dither modulation signal can be adjusted to effectively reduce the background fringe interference noise, which suppress the background interference fringe noise more substantially_ [0044] On the other hand factors affecting In? and at/ in the multi-frequency modulation method include an etalon length, a laser temperature, an average control current, and other system hardware parameters, which are irrelevant to the measurement object. After the hardware system is fixed, these parameters are constant. Therefore, after tri2 and 0/2 are adjusted to the optimal values, there is no need to make further adjustments even for different measurement objects. At this time, the background interference fringes noise of the system has reached the minimum level. The traditional noise suppression algorithm requires more parameters to be changed, and the implementation steps are more complex. When the measurement object is changed, the corresponding parameters need to be adjusted.
DRIEF:DESCRIPTION OF THE DRAWINGS
100451 HG. I is a schematic diagram of a system for measuring oxygen content in a glass bottle by using a TDLAS-WN/S multi-frequency modulation method; [0046] FIG. 2 is a schematic diagram of adjustment of an amplitude mi of a basic modulation signal; [00471 FIG 3 is a schematic diagram of variations of a standard deviation of second harmonic peak data with an amplitude in of a dither modulation signal; [0048] FIG. 4 is a schematic diagram of variations of a standard deviation of second harmonic peak data with a frequency 002 of a dither modulation signal; 1.0049] FIG. 5 shows background interference fringe noise under different modulation systems; and [0050] RIG. 6 shows second harmonic absorption signals under different modulation systems,
DETAILED DESCRIPTION OF THE EMBODIMENTS
[00511 The present disclosure is further described below with reference to embodiments.
[0052] FIG. I is a schematic architecture diagram of a gas concentration measurement system. In this embodiment, a glass bottle is filled with oxygen. Oxygen measurement is taken as an example in this embodiment, and in other feasible embodiments, the gas measurement object may be other gases.
1.0053] The gas concentration measurement system in this embodiment includes: a lock-in amplifier,: a laser driver, a temperature control module, a laser, a glass bottle, a photoelectric detector, a digital oscilloscope" and a digital processor. The laser driver, the photoelectric detector, the digital oscilloscope, and the digital processor are all connected to the Jock-in amplifier, The temperature control module is built in the laser driver. The laser driver and the temperature control module are connected to the laser. The glass bottle is disposed at a laser transmitting end of the laser and is arranged between the laser and the photoelectric detector_ [0054] The lock-in amplifier outputs a driving current signal to the laser drIver, where the driving current signal is formed by low-frequency sawtooth waves, high-frequency sine waves (basic modulation signal) and a dither modulation signal. The laser driver receives the driving current signal, converts the driving current signal into a control current, and coordinates with the temperature control module to cause the laser to work iii a current modulation mode and maintain a laser center wavelength at 760.8 nrit Modulated light emitted by the laser passes through the glass bottle, and is received by the photoelectric detector and converted into an absorption electrical si which is scnt to the lock-in amplifier for demodulation. The second harmonic signal outputted by the lock-in amplifier is finally delivered into the digital processor for data processing; at the same time, the second harmonic signal can also be observed through the digital oscilloscope, to adjust the parameter. The amplitude of the basic modulation signal and the amplitude and frequency of the dither modulation signal are determined according to the following multi-frequency modulation method.
[0055] The second harmonic signal is obtained by superimposing background interference fringes on the second harmonic absorption signal. By reducing the noise caused by the background interference fringes, the second harmonic baseline drift can be suppressed" thereby improving the signal-to-noise ratio and the system measurement accuracy. In the present disclosure, a dither modulation signal is added to the original single-frequency modulation system to constitute a multi-frequency modulation system. After the dither modulation signal is added; the laser driver of the control current is expressed as follows: [0056] )= iff cos mit nt2 cos col [00571 where represents a control current of the laser driver at a current moment is an average current outputted by the laser driver, 1111 is the amplitude of the basic modulation signal,11)1 is the frequency of the basic modulation signal, and initial values of the above parameters are the same as those of the original single--frequency modulation system; a)2 is the frequency of the dither modulation signal, and an initial value thereof is set to 0.5cot; 1i is the amplitude of the dither modulation signal, arid an initial value thereof is set to 0.5m. In other feasible embodiments, the initial values of 192 and 1112 may be set to other values, which are not specifically limited in the present disclosure.
10058i Moreover, after the dither modulation signal is added, thebackground interference hinges may be approximately expressed as follows: [0059] 101: cos ( 4)ivn) (IT AI:)i o (2r. Al 100601 where Alt = 2htfjl = 211en,:f 'o is an average output light intensity of the laser, is an etalon fineness factor, 1 is an etalon length, is a laser frequency-current tuning factor, ro is an average output frequency of the laser, J2(27110 is a second-order Bessel
-
function term, .1-0(27(1112) is a zero-order Bessel function term, and 1" , and -11/1 2 are two variants defined for ease of expression.
[0061] It can be learned from the expression of the background interference fringes that, the background interference fringes can be suppressed by adjusting the 7Th to make Jo equal to zero. Because mi also affects the second harmonic absorption peak signal, tifi is adjusted first such that the horizontal coordinate of the second harmonic peak point is equal to the horizontal coordinate of the envelope minimum point of the background interference fringes,shown in FIG 2, and the value of tnt at this point is selected. The theoretical optimal value of /fl? should cause lo to reach the first zero point. At this point Al2 = 0-383 and the theoretical optimal amplitude value of 1112 can be calculated through 0.383/214. However, the above formula of the background interference fringes is an approximate expression. In practical application, the best option is to adjust the amplitude m2 of the dither modulation signal such that the theoretical optimal amplitude value of m2 varies within a certain range. At the same time, noise levels of the second harmonic peak values under different m2 are analyzed, to find an experimental optimal value of 1112. The frequency (02 of the dither modulation signal does not have a theoretical optimal value. When 02 is set to an integer multiple of col, such as col, 201, or 30)1, the second harmonic signal is generally distorted; when co2<0.21, the background interference fringe noise will increase. Considering the burden of the system hardware, the value of co2 should not be excessively large. Therefore, the value of (02 15 generally within a range of 0.2m1-0./INi. In the embodiment of the present disclosure, preferably,o2 is adjusted within the frequency adjustment range, such that (.02 varies within this range. At the same time, error levels of the second harmonic peak values under different CO2 values are analyzed to find an experimental optimal value of W2. In the embodiment of the present disclosure, adjustment processes of m2 and e02 are as follows: [0062] Within the range of the amplitude adjustment interval [5, +30]12A, the amplitude m2 of the dither modulation signal is increased at an equal step of AiVic--1 ',LA. 500 peak values of the second harmonic absorption curve are collected after each adjustment of the amplitude 1112, then noise levels of the second harmonic peak values under different amplitudes are compared, and finally, an amplitude corresponding to a lowest noise level is selected as the amplitude 12 of the dither modulation signal To exclude interferences such as an environmental factor, this process is preferably repeated KI times, where KI=5, and in other feasible embodiments, KI may be set to other values. In this embodiment, the noise level is a standard deviation of the second harmonic peak values, and in other feasible embodiments, other parameters are selected to represent the noise level, such as an Allan valiance.
[0063] Within the range of the frequency adjustment interval 0.4(ml, the frequency (02 of the dither modulation signal is increased at an equal step of zloic=0.0 I col. .500 peak values of the second harmonic absorption curve are collected after each adjustment of the frequency (.02, then noise levels of the second harmonic peak values under different frequencies are compared, and finally a frequency corresponding to a lowest noise level is selected as the frequency of the dither modulation signal. Similarly, to exclude interferences such as an environmental factor, this process is preferably repeated K2 times, where K and in other feasible embodiments, K2 may be set to other values. In this embodiment, the noise level is a standard deviation of the second harmonic peak values" and in other feasible embodiments, other parameters are selected to represent the noise level, such as an Allan variance.
[0064] Based on the above theoretical description, an embodiment of the present disclosure provides a gas concentration measurement method based on mu ti-frequency modulation, including the following steps: [0065] First, an amplitude of a basic modulation signal is adjusted such that a horizontal coordinate of a peak point of a second harmonic absorption curve outputted by a multi-frequency modulation gas concentration measurement system is the same as a horizontal coordinate of an envelope minimum point of background interference fringes.
[0066] Then, an amplitude of the dither modulation signal is adjusted based on an amplitude adjustment interval of the dither modulation signal. This embodiment is executed based on the process described above. In other feasible embodiments, the amplitude may also be selected within the amplitude adjustment interval.
[0067] Finally, a frequency of the dither modulation signal is adjusted based on a. frequency adjustment interval of the dither modulation signal. This embodiment is executed based on the process described above. In other feasible embodiments; the frequency may also be selected within the frequency adjustment interval.
[0068] it should be noted that, in this embodiment, the amplitude of the basic modulation signal is adjusted first, the the amplitude of the dither modulation signal is adjusted, and finally the frequency of the dither modulation signal is adjusted. In other feasible embodiments, the amplitude of the basic modulation signal may be adjusted first, and the adjustment order of the amplitude of the dither modulation signal and the frequency of the dither modulation signal is not limited.
[0069] Factors affecting 3/22 and 002 in the multi-frequency moduia,ton method include etalon length, laser temperature, average control current, and other system hardware parameters, which are irrelevant to the measurement object. After the hardware system is fixed, these parameters are constant. Therefore, after 1/22 and cuz are adjusted to the optimal values, there is no need to make further adjustments even for different measurement objects. At this time, the background interference fringes noise of the system has reached the minimum level. Therefore, during switching between different measurement objects, the multi-frequency modulation method includes: 100701 first building a gas concentration measurement system determined by a previous gas; to measure a concentration of a new gas object; and [0071j adjusting an amplitude of a basic modulation signal such that a horizontal coordinate of a peak point of a second harmonic absorption curve outputted by the gas concentration measurement system is the same as a horizontal coordinate of an envelope mmimnumn point of current background interference fringes.
[0072] it can be learned from the above that, while the system hardware platform remains unchanged; tm and cur do not need to be adjusted when a measurement object is changed; and only the amplitude nri of the basic modulation signal needs to be adjusted for the different measurement object.
[0073] In addition, the multi-frequency modulation method provided by the present disclosure is applied to oxygen measurement The present disclosure provides a gas concentration measurement method based on the above multi-frequency modulation method, including: [0074] adjusting an amplitude of a basic modulation signal and an amplitude and a frequency of a dither modulation signal of a system by using the above multi-frequency modulation method; and [0075] measuring a gas concentration based on a modulated gas concentration measurement system It should be noted that, this step is implemented with a known concentration measurement method.
[0076] It should be noted that, for the gas concentration measurement system, in some embodiments, after the amplitude of the basic modulation signal and the amplitude and frequency of the dither modulation signal are determined according to the above method, the amplitude of the basic modulation signal and the amplitude and frequency of the dither modulation signal can be adjusted or changed by manually setting a parameter of the lock-in amplifier.
[0077] For the gas concentration measurement system, in some embodiments, the amplitude of the basic modulation signal and the amplitude and frequency of the dither modulation signal may also be adjusted automatically. In this case; the digital processor executes instmctions to implement the following operations: [0078] Identifying whether a hotizontal coordinate of a peak point of a second harmonic absorption curve is equal to a horizontal coordinate of an envelope minimum point of background interference fringes according to the second harmonic signal; if yes, feeding back a signal to the lock-in amplifier, and keeping the current amplitude of the basic modulation signal unchanged; otherwise, feeding back an amplitude adjustment instruction for the basic modulation signal to the lock-in amplifier, such that the lock-in amplifier performs adjustment, where after each adjustment, the digital processor makes a judgment until the horizontal coordinate of the peak point of the second harmonic absorption curve is equal to the horizontal coordinate of the envelope minimum point of the background interference fringes; and [0079i feeding back an amplitude adjustment instruction and a frequency adjustment instruction for the dither modulation signal to the lock-in amplifier based on an amplitude adjustment interval and a frequency adjustment interval of the dither modulation signal, such that the lock-in amplifier performs adjustment. A -theoretical optimal amplitude value of the dither modulation signal is at least within the amplitude adjustment interval, and is obtained when a zero-order Bessel function reaches a first zero point, where = 0.383 / 2/,-; The amplitude adjustment interval is preferably: [5, Al-+30 nA, and j'-d is the theoretical optimal amplitude value of the dither modulation signal; and the frequency adjustment interval is: [0.2coi, 0.40iid; and col is a frequency of the basic modulation signal.
10080! Further., the digital processor executes instructions to implement the following o pecan on s [0081] adjusting the amplitude of the dither modulation signal at an equal step of alivic within the amplitude adjustment interval; collecting N peak values of the second hainionic absorption curve after each amplitude adjustment; then comparing noise levels of the peak values of the second harmonic absorption curve under different amplitudes, and finally selecting an amplitude corresponding to a lowest noise level as the amplitude of the dither modulation signal, where N is a positive integer; and [0082] adjusting the frequency of the dither modulation signal at an equal step of AUK; within the frequency adjustment interval, collecting N peak values of the second harmonic absorption curve after each frequency adjustment, then comparing noise levels of the peak values of the second harmonic absorption curve under different frequencies, and finally selecting a frequency corresponding to a lowest noise level as the frequency of the dither modulation signal.
[0083] In another embodiment, the present disclosure provides a gas concentration measurement apparatus based on multi-frequency modulation; including a lock-in amplifier, a digital processor, and a memory that are connected to one another. The lock-in amplifier outputs a driving current signal to a laser driver of a gas concentration measurement system; receives an absorption electrical signal from a photoelectric detector in die gas concentration measurement system, and outputs a second harmonic signal to the digital processor after demodulation, where the driving current signal is formed by low-frequency sawtooth waves, a basic modulation signal, a.nd a dither modulation signal. The digital processor is connected to the memory storing instructions, and the digital processor executes the instructions stored in the memory to implement the following operations: [0084] identifying whether a horizontal coordinate of a peak point of a second harmonic absorption curve is equal to a horizontal coordinate of an envelope minimum point of background interference fringes according to the second harmonic signal; if yes, feeding back a signal to the lock-in amplifier, and keeping the current amplitude of the basic modulation signal unchanged; otherwise, feeding back an amplitude adjustment instruction for the basic modulation signal to the lock-in amplifier,such that the lock-in amplifier performs adjustment, where after each adjustment; the digital processor makes a judgment until the horizontal coordinate of the peak point of the second harmonic absorption curve is equal to the horizontal coordinate of the envelope minimum point of the background interference fringes; and [0085] feeding back an amplitude adjustment instruction and a frequency adjustment instruction for the dither modulation signal to the lock-in amplifier based on an amplitude adjustment interval and a frequency adjustment interval of the dither modulation signal, such that the lock-in amplifier performs adjustment A theoretical optimal amplitude value of the dither modulation signal is at least within the amplitude adjustment interval, and is obtained when a zero-order Bessel function term reaches a:first zero point where Ald =°-383 /2ke-. The amplitude adjustment interval is preferably: [5, A Id +30] }LA; and Mor is the theoretical optimal amplitude value of the dither modulation signal; and the frequency adjustment interval is: [0.2rni. 0.4o)d, and WI is a frequency of the basic modulation signal.
[0086] Further, the digital processor executes the instructions stored in the memory to implement the following operations: [0087] adjusting the amplitude of the dither modulation signal at an equal step of zlivic within the amplitude adjustment interval, collecting N peak values of the second harmonic absorption curve after each amplitude adjustment, then comparing noise levels of the peak values of the second harmonic absorption curve under different amplitudes, and finally selecting an amplitude corresponding to a lowest noise level as the amplitude of the dither modulation signal, where Nis a positive integer; and 1.00823] adjusting the frequency of the dither modulation signal at an equal step of Awe within the frequency adjustment interval, collecting N peak values of the second harmonic absorption curve after each frequency adjustment, then comparing noise levels of the peak values of the second harmonic absorption curve under different frequencies, and finally selecting a frequency corresponding to a lowest noise level as the frequency of the dither modulation signal.
[0089] It should be understood that, instructions invoked by the digital processor may he stored in an internal memory unit of the digital processor or may be stored in an external memory device. That is, the memory may be an internal memory unit or an external memory device of the digital processor, which is not specifically limited in the present disclosure. The digital processor may be a central processing unit (CPU), and or another general--purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA) or another programmable logic device; a discrete gate, a transistor logic device, a discrete hardware component, etc. The general-purpose processor may be a microprocessor, or the processor may be any conventional processor [0090] if ordinary skill in the art may realize that the unit elements and the algorithm steps of the examples described in miniodinierits of the present disclosure can he implemented by electronic hardware, computer software, or a combination thereof. In order to clearly describe the interchangeability between the hardware and the software, compositions and steps of each example have been generally described according to functions in the foregoing descriptions. Whether these functions are implemented in hardware or software depends on specific applications of the technical solutions and design constraints. A person skilled in the art may use dtferent methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of the present disclosure.
[0091] A person skilled in the art can clearly understand that, for convenience and brevity of description, reference can be made to corresponding processes in the foregoing method einbodiinents for specific: working processes of the above-described hardware elements. Details are not described herein again. In the embodiments provided by the present disclosure, the intercoupling of direct coupling or communication connection shown or discussed may be indirect coupling or communication connection through some interfaces; apparatuses, of units; or may be implemented in electrical, mechanical; or other forms.
[0092] In order to experiment the method described in the present discloser, the experiment was conducted as follows: [0093] Main parameters involved in the experiment are as follows: a center frequency v of a laser is 13143.2 cm-1, a center frequency ve of art oxygen spectral line is 13142.6 cm4 an atmospheric pressure P is 1 atm, a laser temperature T is 296 K, an open optical range length L is 30 cm; an oxygen concentration X in a glass bottle is 21%; a laser frequency-current tuning factor is 0.6 cm-l/mA, an etalon length is 20 cm; and a fineness factor F is 0.08. A frequency f of scanning sawtooth waves is 25 Hz, an amplitude in of the scanning sawtooth waves is 10 mA, a DC bias k of the scanning sawtooth waves is 2 mA, a frequency co! of a basic modulation signal is 2500 Hz, and an initial value of an m of the basic modulation signal is set to 0.14 mA, an initial value of a frequency (02 of a dither modulation signal is set to 0.5oit=1250 Hz, and an initial value of an amplitude ml of the dither modulation signal is set to 0.5m/=-20!IA Parameter optimization for ini,m2, co2 was carried out as follows: [0094] The value of En; was fine-tuned carefully, At the same time, a position change of a second harmonic absorption peak point in an oscilloscope relative to an envelope minimum point of background interference fringes was observed. If the peak point moved away from the envelope minimum point, the value of fiti was adjusted reversely until the position of the second harmonic absorption peak coincides with the position of the envelope minimum point of the background interference fringes, as shown in FIG. 2. hi this case, int=0.12 inA. The value was fixed; and then in? was adjusted.
10095.1 By using the formula in/ = 0.383 / 2i4.:. and the experimental parameters, a theoretical optimum value of /n2 was calculated to be 16 RA, en2 wa.s adjusted to increase within the range of [5 pA, 46 pA] at a step of I p.A. At the same time, 500 second harmonic absorption peak points ware measured at each value of /122, and a standard deviation of these 500 data points was calculated and recorded. To exclude the impact of environmental fluctuations, this procedure was repeated five times, corresponding to experiments 1 to experiment 5 in FIG. 3. FIG. 3 shows variations of the standard deviation with 11/2 for the recorded second harmonic peak data. In the figure, the curve of experiment 3 has the minimum value at in2=1.8 pA, and the curves of the other four experiments have the minimum value at m2=17 pA. Both values are close to the theoretical optimum. According to the majority principle, m2=17 pA was selected, and the value was fixed.
[0096] Finally, CD? was adjusted to increase within the range of [500 Hz, 1000 Hz] at a step of 25 Hz. At the same time, 500 second harmonic absorption peak points were measured at each value of co../, and a standard deviation of these 500 data points was calculated and recorded. To exclude the impact of environmental fluctuations, this procedure was repeated twice, corresponding to experiment 1 and experiment 2 in FIG. 4. FIG. 4 shows variations of the standard deviation with 022 for the recorded second harmonic peak data. The curves of the two experiments in the figure overlap well, and the second harmonic signal has good stability at 500 Hz, 550 Hz, 575 Hz, 600 Hz, 750 I-14, 800 Hz, 900 Hz, 950 Hz, 1000 Hz, and so on Considering the hardware burden, the low frequency is usually selected. Therefore, ai2=500 Hz.
100971 After the above steps, the parameters of the multi-frequency modulation system have been optimized. The noise levels of the background interference fringes of the single-frequency modulation system and the multi-frequency modulation system are compared. FIG. 5 shows the 2f signal of the background interference fringes without gas absorption. It can be seen that the background interference fringe noise under single-frequency modulation is significantly higher than the background interference fringe noise under multi-frequency modulation, and significant baseline drift occurs. It is obtained through calculation that under single-frequency modulation, the mean value of the background interference fringe noise is 5.07, the variance is 2.85, and the ratio of the variance to the mean value is 56.2%; under multi-frequency modulation, the mean value of the background interference fringe noise is 0.084, the variance is 0.0048, and the ratio of the variance to the mean value is 5.7%. FIG. 6 shows that the 2f absorption signal after multi-frequency modulation fits well with the 2f absorption signal in the ideal case. The 2f absorption signal in the single-frequency system has distortion and baseline drift which greatly affect the measurement accuracy and precision of the system. A signal-to-noise ratio (SNR) of the 2f absorption signal in the single-frequency system is 3 122 dB" and the SNR of the 2f absorption signal after multi47requency modulation is 36 979 dB" which is an order of magnitude higher In summary, it is verified that the method can effectively suppress the interference fringe noise and baseline drift, and improve the SNR and stability of the system.
[0098] It should be emphasized that the embodiment in the present disclosure is illustrative rather than restrictive. Therefore, the present disclosure includes but is not limited to the embodiment in the detailed description. All other implementations such as modifications or replacements obtained by those skilled in the art according to the technical solutions of the present disclosure without departing from the spirit and scope of the present disclosure shall also fall within the protection scope of the present disclosure.
Claims (6)
- WHAT ES CLAIMED IS: I. A gas concentration measurement method based on multi-frequency modulation, comprising: step Si: adjusting an amplitude of a basic modulation signal such that a horizontal coordinate of a peak point of a second harmonic absorption curve outputted by a muiti-frequency modulation gas concentration measurement system is the same as a horizontal coordinate of an envelope minimum point of background interference fringes, wherein a dither modulation signal is added to a single-frequency modulation gas concentration measurement system based on tunable diode laser absorption spectroscopy (TDLAS)-wavelength modulation spectroscopy (WN4S), to obtain the multi-frequency modulation gas concentration measurement system; a second harmonic signal outputted by the multi-frequency modulation gas concentration measurement system is formed by a second harmonic absorption signal superimposed with the background interference fringes; and an initial frequency value and an initial amplitude value of the added dither modulation signal are preset values: and step S2: adjusting an amplitude and a frequency of the dither modulation signal based on an amplitude adjustment interval and a frequency adjustment interval clf the dither modulation signal.
- 2. The method according to claim I, wherein a theoretical optimal amplitude value of the dither modulation signal is at least within the amplitude adjustment interval, and is obtained when a zero-order Bessel function term reaches a first zero point.
- 3. The method according to claim 2, wherein the theoretical optimal amplitude value of the dither modulation signal is as follows: At= 0.383 / 21: wherein Aid is the theoretical optimal amplitude value of the dither modulation signal, I is an etalon length, and is a laser frequency-current tuning factor.
- 4. The method according, to claim 2, wherein the amplitude adjustment interval is: [5, Al:t +30] pA, and -Aid is the theoretical optimal amplitude value of the dither modulation signal; and the frequency adjustment interval is: [0.2ail, and on is a frequency of the basic modulation signal.
- S. The method according to claim 1, wherein the process of adjusting an amplitude and a frequency of the dither modulation signal based on an amplitude adjustment interval and a frequency adjustment interval of the dither modulation signal is as follows: adjusting the amplitude of the dither modulation signal at an equal step of ziAle within the amplitude adjustment interval, collecting N peak values of the second harmonic absorption curve after each amplitude adjustment, then comparing noise levels of the peak values of the second harmonic absorption curve under different amplitudes, and finally selecting an amplitude corresponding to a lowest noise level as the amplitude of the dither modulation signal:, wherein N is a positive integer; and adjusting the frequency of the dither modulation signal at an equal step of Awe within the frequency adjustment interval, collecting N peak values of the second harmonic absorption curve after each frequency adjustment; then comparing noise levels of the peak-values of the second harmonic absorption curve under different frequencies, and finally selecting a frequency corresponding to a lowest noise level as the frequency of the dither modulation signal.
- 6. A gas concentration measurement method based on multi-frequency modulation, wherein when a concentration of another gas object is measured by a gas concentration measurement system with an amplitude of a basic modulation signal and an amplitude and a frequency of a dither modulation signal determined by using the method of claim I, the multi-frequency modulation. method comprises: building the gas concentration measurement system dete mined in claim to measure a concentration of another gas object; and a.djusting the amplitude of the basic modulation signal such that a horizontal coot ate of a. peak point of a second harmonic absorption curve outputted by the gas concentration measurement system is the same as a horizontal coordinate of an envelope minimum point of current background interference fringes.concentration measurement method, comprising: adjusting amplitude of a basic modulation signal and an amplitude and a frequency of a dither modulation signal by using the method according, to claim 1 or claim 8, and measuring a gas concentration based on a modulated gas concentration measurement system 8, A gas concentration measurement system, comprising: lock-in amplifier, a. laser driver, a temperature control module, a laser, a glass bottle, and a photoelectric detector, wherein the laser driver and the photoelectric detector are both connected to the lo * amplifier, the temperature control module is built in the laser driver; the laser driver and the temperature control module are both connected to the laser, the glass bottle is disposed at a laser transmitting end of the laser and is arranged between the laser and the photoelectric detector, and the glass bottle contains a gas whose concentration is to be measured; the lock-in amplifier outputs a driving current signal to the laser driver, wherein the driving current signal is formed by low-frequency sawtooth waves, a basic modulation signal, and a dither modulation signal, and an amplitude of the basic modulation signal and an amplitude and a frequency of the dither modulation signal are determined by using the method according to claim 1 or claim 8; the laser driver generates a control current after receiving the driving current signal from the lock-in amplifier; and jointly controls, with the temperature control module, the laser to emit modulated light; and the modulated light emitted by the laser is received by the photoelectric detector after passing through the glass bottle, the photoelectric detector converts an optical signal into an absorption electrical signal and transmits the absorption electrical signal to the lock-in amplifier for demodulation, and the lock-in amplifier outputs a second harmonic signal after dem odul au on 9. The detection system according to claim 82 further comprising a digital processor, wherein the digital processor is connected to the lock-in amplifier, and the lock--in amplifier outputs the second harmonic signal to the digital processor; and the digital processor executes the following instmctions: vhether a horizontal coordinate of a peak point of armonic absorption curve is equal to a horizontal coordinate of an envelope minimum point of background interference fringes according to the second harmonic signal; if yes, feeding back a signal to the lock-in amplifier, and keeping the current amplitude of the basic modulation signal unchanged, otherwise, feeding back an amplitude adjustment instruction for the basic modulation signal to the lock-in amplifier, such that the lock-in amplifier performs adjustment; and feeding back an amplitude adjustment instruction and a frequency adjustment instruction for the dither modulation signal to the lock-in amplifier based on an amplitude adjustment interval and a frequency adjustment interval of the dither modulation signal, such that the lock-in amplifier performs adjustment.10. A gas concentration nleasLlrenlent apparatus based on multi-frequency modulation, I9 comprising a lock-in amplifier, a digital processor, and a memory that are connected to one another, wherein the lock-in amplifier outputs a driving current signal to a laser driver of a gas concentration measurement system, receives an absorption electrical signal from a photoelectric detector in the gas concentration measurement system, and outputs a second harmonic signal to the digital processor after demodulation, wherein the driving current signal is formed by low-frequency sawtooth waves, a basic modulation signal, and a dither modulation signal; and the digital processor is connected to the memory storing instructions, and the digital processor executes the instructions stored in the memory to implement the following operations: identifying whether a horizontal coordinate of a peak point of a second harmonic absorption curve is equal to a horizontal coordinate of an envelope minimum point of background interference fringes according to the second harmonic signal; if yes, feeding back a signal to the lock-in amplifier, and keeping the current amplitude of the basic modulation signal unchanged; otherwise, feeding back an amplitude adjustment instruction for the basic modulation signal to the lock-in amplifier, such that the lock-in amplifier performs adjustment; and feeding back an amplitude adjustment instruction and a frequency adjustment instruction for the dither modulation signal to the lock-in amplifier based on an amplitude adjustment interval and a frequency adjustment interval of the dither modulation signal, such that the lock-in amplifier performs adjustment.
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| CN114199820B (en) * | 2021-12-07 | 2024-06-07 | 北京华亘安邦科技有限公司 | Method and device for detecting gas concentration |
| CN114705654B (en) * | 2022-04-01 | 2022-11-18 | 华电智控（北京）技术有限公司 | Gas concentration measuring method and device compatible with optical interference effect |
| CN114942236B (en) * | 2022-05-07 | 2024-07-05 | 山西大学 | Method for eliminating influence of light intensity jitter on laser gas telemetry |
| CN115773996A (en) * | 2022-11-29 | 2023-03-10 | 中国电子科技集团公司第四十八研究所 | High-reliability laser oxygen partial pressure detection device and using method and correction method thereof |
| CN115791698B (en) * | 2023-02-02 | 2023-05-23 | 合肥清芯传感科技有限公司 | Gas concentration detection apparatus, control method thereof, control device thereof, and storage medium |
| CN115963081B (en) * | 2023-03-16 | 2023-08-01 | 杭州泽天春来科技有限公司 | Gas analyzer |
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