CN202661377U - Laser infrared gas analyzer based on TDLAS-WMS (tunable diode laser absorption spectroscopy-wavelength modulation spectroscopy) - Google Patents
Laser infrared gas analyzer based on TDLAS-WMS (tunable diode laser absorption spectroscopy-wavelength modulation spectroscopy) Download PDFInfo
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- CN202661377U CN202661377U CN201220257331XU CN201220257331U CN202661377U CN 202661377 U CN202661377 U CN 202661377U CN 201220257331X U CN201220257331X U CN 201220257331XU CN 201220257331 U CN201220257331 U CN 201220257331U CN 202661377 U CN202661377 U CN 202661377U
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- 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
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- 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
- G01N2021/399—Diode laser
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Abstract
The utility model belongs to the technical field of gas detection and relates to a laser infrared gas analyzer based on TDLAS-WMS (tunable diode laser absorption spectroscopy-wavelength modulation spectroscopy) for detecting gas such as hydrogen chloride, methane, carbon monoxide and water vapor. The laser infrared gas analyzer comprises a laser, a laser driving circuit, a temperature control circuit, an optical system, a main detector, a reference detector, an intensity modulation eliminating circuit, a phase-locked amplifying circuit and a data collecting and display circuit, wherein the optical system is provided with an optical cavity, the laser driving circuit and the temperature control circuit are used for controlling the laser to give out light, two ends of the optical system are respectively connected with the laser and the detector, the intensity modulation eliminating circuit is used for eliminating the influence of the intensity modulation in the system, the phase-locked amplifying circuit is used for extracting harmonic signals, and the data collecting and display circuit is used for displaying the concentration of gas to be measured. Compared with other detection instruments, the laser infrared gas analyzer has the advantages that the division operation is introduced in the intensity modulation eliminating circuit, a space double-light-path differential detection method is combined, and the influence of the intensity modulation can be fundamentally eliminated.
Description
Technical field
The utility model belongs to the gas detection technology field, be specifically related to a kind of to the gases such as hydrogen chloride, methane, carbon monoxide and water vapor detect based on TDLAS-WMS(Tunable Diode Laser Absorption Spectroscopy, TDLAS, Wavelength modulation spectroscopy, WMS) the laser infrared gas analyser.
Background technology
Hydrogen chloride is colourless and the gas of penetrating odor is arranged.Volatile, diffusivity is strong, and is soluble in water.Hydrogen chloride also is a kind of important industrial chemicals, therefore, in chemical process, can produce inevitably the waste gas of hydrogen chloride, because hydrogen chloride has the strong impulse effect to people's eyes and respiratory system, can cause a series of human body diseases even death.Hydrogen chloride also has certain harm to environment, in order to reduce as much as possible hydrogen chloride gas the mankind's production and life is impacted, and is most important to its monitoring.
Methane distributes very wide at occurring in nature, it is the important gas in the greenhouse effect, greenhouse effect have very large impact to biological and environment of living in, methane also is the principal ingredient of methane gas in the coal production, in recent years, the China's coal-mine explosion accident happens occasionally, and has caused the massive losses that much can't retrieve.So the detection to methane also is duty-bound.
Carbon monoxide is the widest and maximum pollutant of quantity that distributes in the atmosphere, also is one of important pollutant that generates in the combustion process.Carbon monoxide has toxicity, carbon monoxide enter after the human body can and blood in the haemoglobin combination, thereby make haemoglobin can not with combination with oxygen, cause that anoxic appears in body tissue, cause human body to suffocate even death.Because carbon monoxide is colourless, tasteless gas, is easy to be ignored by people and causes poisoning.So pay attention to Carbon Monoxide Detection.
And water vapor is that humidity and daily life are closely related, and simultaneously humidity also has important impact to every field, therefore, the detection of water vapor also is significant.
The essence of infrared gas analyzer is to utilize specific gas molecule that specific infrared spectrum is produced these characteristics of absorption, damping capacity and the gas concentration of light after by gas is connected, and then concentration signal is changed into light signal, finally changes electric signal into.Adopt tunable diode laser absorption spectroscopy (Tunable Diode Laser Absorption Spectroscopy, TDLAS) in conjunction with Wavelength modulation spectroscopy (Wavelength modulation spectroscopy, WMS) technology is measured gas concentration, has highly sensitive, the advantages such as selectivity good, long-time stability.Its ultimate principle is: utilize the electric current tuning of laser instrument and the characteristic of thermal tuning, make the output wavelength of laser instrument scan a certain absorption peak of gas to be measured, simultaneously laser instrument is modulated, detect according to the correlativity of harmonic signal and gas concentration.But this technology is existent defect also: the Injection Current of adjusting laser instrument will cause the variation of laser power when realizing length scanning and modulation, namely produce the intensity modulated phenomenon, thus the precision that impact is measured.
In order to eliminate the intensity modulated phenomenon, generally adopt the harmonic wave ratioing technigue.Because the harmonic wave peak value diminishes with the rising of order, thereby usually utilizes the first harmonic signal that second harmonic signal is carried out normalized.Although this way can be eliminated the public keys such as light intensity, optical gain, can't fundamentally eliminate the intensity modulated phenomenon.But also the phase differential, modulation that need measured intensity modulation and wavelength-modulated have increased the complexity of system once with the quadratic term coefficient etc.In addition, the waveform of first harmonic is unstable, with the detection method meeting of second harmonic and first harmonic ratio so that the concentration value that records is more unstable, thereby increase the error of measuring.
The utility model content
The purpose of this utility model is according to infrared absorption principle (Lambert-Beer's law), detects hydrogen chloride, methane, carbon monoxide and water vapor concentration based on the TDLAS-WMS method.The utility model has proposed a kind of new solution for the intensity modulated phenomenon that exists in measuring, and namely utilizes division arithmetic in conjunction with Space Double light path difference detecting method, fundamentally eliminates the intensity modulated phenomenon, overcomes the shortcoming of conventional method.
The technical scheme that the utility model adopts is with the TDLAS(tunable diode laser absorption spectroscopy) with the WMS(Wavelength modulation spectroscopy) combine.By changing the emission wavelength of laser instrument, making the absorption jump spectral line of its spectral scan gas to be measured in itself.In experiment, at first by adjusting the working temperature of laser instrument, make the optical wavelength of output of laser instrument near the centre wavelength position of absorption jump, then, regulate the electric current of drive laser again to further meticulous adjusting of wavelength, purpose be make the laser instrument emission centre wavelength as far as possible with the absorption line centrally aligned.After having determined working temperature and dc bias current, DC current is periodically slowly changed, to realize that optical maser wavelength is to the scanning of whole transition spectral line.In order to reduce the 1/f noise in the system, the proportion modulation technique arrives high-frequency region with the measured signal frequency modulation (PFM); In order to eliminate the interference of the factors such as intensity modulated and light-intensity variation, the technology that has adopted division arithmetic to combine with general Space Double light path Differential Detection method; Adopting phase lock amplifying technology is in order to extract second harmonic signal, according to the linear relationship of second harmonic signal and gas concentration, realizes the detection to gas concentration.
A kind of laser infrared gas analyser based on TDLAS-WMS of the present utility model, by laser instrument (inner integrated laser diode, a thermistor and a thermoelectric refrigerating unit), drive circuit for laser, temperature-control circuit, optical system with optical cavity, two infrared eyes (main detector and a reference detector), intensity modulated is eliminated circuit, phase-locked amplifying circuit and data acquisition and display the electric circuit constitute, drive circuit for laser connecting laser wherein, and laser instrument modulated with tuning, make the output wavelength of laser instrument can scan the absorption jump spectral line of gas to be measured, also reduced the 1/f noise of system simultaneously; Temperature-control circuit is connected with laser instrument, and can regulate the temperature of laser instrument in 0 ℃～70 ℃ scopes, prevents that laser instrument from damaging because of too high or too low for temperature; End connecting laser with the optical system of optical cavity, the other end connects infrared eye, the light that laser instrument sends is divided into two bundles in optical system, one the tunnel through optical cavity by the gas absorption to be measured in the optical cavity after, be sent to main detector, the one tunnel directly is sent to reference detector; Infrared eye is transformed into current signal with the light signal that receives; Intensity modulated is eliminated circuit and is connected infrared eye, convert the current signal of infrared eye output to voltage signal, the technology of utilizing simultaneously division arithmetic to combine with general Space Double light path Differential Detection method, the variation of the laser power that elimination causes because of length scanning and modulation, namely intensity modulated is on the impact of voltage signal; Phase-locked amplifying circuit strength of joint modulation eliminating circuit mainly is to extract second harmonic signal, and with about the signal magnify 20; The data acquisition and display circuit connects phase-locked amplifying circuit, according to the peak value of second harmonic signal and the linear corresponding relation of gas concentration, utilize minimum mean square error criterion, the concentration value of gas to be measured in second harmonic signal peak value and the optical cavity is carried out match, finally show the gas concentration that records by display circuit.
Wherein, laser instrument is distributed feedback semiconductor quantum cascade laser or distributed feedback semiconductor quantum-well laser, and it also comprises a thermistor and a thermoelectric refrigerating unit TEC; Thermistor is as temperature sensitive member, the temperature of induced quantum cascade laser or quantum-well laser.Thermoelectric refrigerating unit is mainly used in changing the temperature of quantum cascade laser or quantum-well laser as temperature control component.
Wherein, drive circuit for laser comprises Waveform generating circuit, adding circuit and controlled constant-current source circuit.The output terminal of Waveform generating circuit connects adding circuit, adding circuit is with the voltage signal stack of Waveform generating circuit output, the output terminal of adding circuit is connected with controlled constant-current source circuit, the output terminal connecting laser of controlled constant-current source circuit, and drive laser is luminous.
Based on above-mentioned, Waveform generating circuit comprises circuit for generating triangular wave and sine wave generation circuit, produce the triangular wave of length scanning and the sine wave of wavelength-modulated, also produce simultaneously two respectively with triangular wave and sinusoidal wave with square wave frequently, as the reference frequency of trigger source and the phase-locked amplifying circuit of data acquisition, utilize monolithic integrated function generator ICL8038 to realize triangular wave and sine wave generation circuit.
Wherein, temperature-control circuit comprises temperature acquisition and control module, temperature display unit, keyboard unit, temperature storage unit, temperature acquisition and control module connect respectively temperature display unit, keyboard unit and temperature storage unit as the core of temperature-control circuit.
Wherein, optical system is formed by connecting by fiber adapter, optoisolator, beam splitter, optical attenuator, collimating apparatus, optical cavity, optical fiber.The light that laser instrument sends enters optical system through fiber adapter, and an end of fiber adapter connects optoisolator, and the other end of optoisolator connects beam splitter; An output terminal of beam splitter connects optical cavity by collimating apparatus, and the output of optical cavity connects fiber adapter by collimating apparatus, and this fiber adapter connects the infrared eye (main detector) of optical path; Another output terminal of beam splitter connects optical attenuator, and optical attenuator connects another fiber adapter, and this fiber adapter connects the infrared eye (reference detector) of reference path, wherein connects by optical fiber between the optical system each several part.
Based on above-mentioned, optical cavity is a rectangular parallelepiped, height is identical with width, indoor design is cylindrical, there are an air intake opening (external thread), a gas outlet (external thread) and a barometer interface (internal thread) in the top, in order to prevent corrosion and get rusty that whole air chamber adopts stainless steel material to make.
Wherein, intensity modulated is eliminated circuit and is comprised photoelectric switching circuit, subtraction circuit, division circuit, synchronizing signal summation circuit, photoelectric switching circuit connects infrared eye, the output of photoelectric switching circuit connects subtraction circuit, the output of subtraction circuit connects division circuit, and the output of division circuit connects the synchronizing signal summation circuit.Photoelectric switching circuit converts current signal to voltage signal, the Main Function of subtraction circuit is that the difference of extraction voltage signal to be measured (main detector) and reference voltage signal (reference detector) is variable quantity, and then elimination ground unrest, and division circuit carries out division arithmetic to voltage signal difference and reference signal value, further eliminates the interference of the fluctuation introducing of light source self fluctuation and driving circuit.
The utility model is introduced the synchronizing signal summation circuit, then is to utilize the periodicity of measured signal and the randomness of noise, samples periodically with the sampling integrator that the sampling time is extremely short.Like this, for measured signal, because the interval set time, the numerical value of at every turn adopting is identical, and its integral mean still is this signal instantaneous value this moment; And for noise signal, because it has randomness, along with the increase of sampling number, its mean value will diminish.Thereby weakened random noise, improved widely signal to noise ratio (S/N ratio).Periodic noise disturbs then suppressed in the coherent detection process, and cycle and aperiodicity noise have all obtained weakening like this.
Wherein, phase-locked amplifying circuit mainly comprises phase-sensitive detection circuit and low-pass filter circuit.The output of phase-sensitive detection circuit connects low-pass filter circuit, the input end of the output connection data acquisition and display circuit of low-pass filter circuit.Phase-locked amplifying circuit utilizes measured signal relevant with reference signal, and the incoherent characteristic of noise and reference signal has suppressed noise well.
Wherein, the data acquisition and display circuit comprises modulus (Analog to Digital, A/D) change-over circuit, key control circuit, microcontroller (Micro Control Unit, MCU, single-chip microcomputer) and display circuit, wherein microcontroller connects respectively A/D change-over circuit, key control circuit and display circuit.
The beneficial effects of the utility model are, when detecting gas concentration, adopt simple method to eliminate the impact of intensity modulated, have improved largely the precision of measuring.
The utility model adopts phase lock amplifying technology, and phase-locked amplification is based on the signal processing method of correlation detection technology, can effectively extract feeble signal.
The utlity model has temperature-control circuit, can effectively control the temperature of laser instrument, avoid because of the infrared light supply that damages too high or too low for temperature.
The utility model is powered to laser instrument and detector respectively, and avoiding affects the infrared light strong band because of the unstable of supply voltage.
The utility model is when measuring gas concentration to be measured, the gas of getting the concentration known in the certain limit, then according to the peak value of second harmonic signal and the linear corresponding relation of gas concentration, utilize minimum mean square error criterion, the second harmonic signal peak value and the gas concentration value that record are carried out match, obtain the relation curve of second harmonic signal peak value and gas concentration value.When carrying out concrete measurement, according to the second harmonic signal peak value and the relation curve that record, linear interpolation finally obtains the concentration of gas to be measured.
Description of drawings
Fig. 1: the one-piece construction figure of infrared gas analyzer;
Fig. 2: the driving circuit figure of laser instrument;
Fig. 3: laser temperature control circuit structural drawing;
Fig. 4: optical system configuration composition;
Fig. 5: optical cavity structural drawing;
Fig. 6: intensity modulated is eliminated circuit diagram;
Fig. 7: phase-locked amplification circuit diagram;
Fig. 8: data acquisition and display circuit diagram;
Fig. 9: methane 2v
3The fine structure map of the R of general frequency band (3) band;
Figure 10: the graph of a relation of second harmonic signal and gas concentration.
Embodiment
Now by reference to the accompanying drawings embodiment of the present utility model is described in detail.
Fig. 1 is the one-piece construction synoptic diagram of infrared gas analyzer; Wherein circuit for generating triangular wave 1, sine wave generation circuit 2, and adding circuit 3, current voltage-controlled constant-current circuit 4 are driving circuits of laser instrument 6, the control laser instrument is luminous; The temperature that temperature-control circuit 5 is regulated laser instrument by the thermistor in the laser instrument 6 and thermoelectric refrigerating unit avoids laser instrument to damage because of too high or too low for temperature; The light that sends from laser instrument 6 is sent in main detector 8 and the reference detector 9 through optical system 7, eliminates circuit 10 by intensity modulated the light signal that receives is converted to voltage signal; Voltage signal passes through phase-locked amplifying circuit 11 successively, A/D circuit 12, and the processing of single-chip microcomputer (MCU) circuit 13, the concentration value that records the most at last shows at liquid crystal 14.
The physical circuit figure of the drive circuit for laser of above-mentioned infrared gas analyzer as shown in Figure 2.
The scanning triangular wave makes the absorption jump spectral line of its spectral scan gas to be measured by the emission wavelength that changes laser instrument; Modulated sinusoid utilizes frequency modulating technology that high-frequency region is arrived in the measured signal frequency modulation (PFM), and according to the characteristic of 1/f noise, frequency is higher, and noise is less, thereby reaches the effect that reduces 1/f noise.That the generation of scanning triangular wave and modulated sinusoid is used is integrated function generator ICL8038 chip (U1 and U2), only needs peripheral components seldom just can produce triangular wave (output of 3 pin), sinusoidal wave (output of 2 pin), square wave (output of 9 pin) in the frequency range of 0.001Hz～300kHz.Its output signal frequency by 4 pin with are connected the electric capacity that resistance that pin connects and 10 pin connect and jointly determine.
Wherein Fig. 2 (a) is sine wave generation circuit, and sinusoidal wave output amplitude is 2.63V, and frequency is 5kHz, and the chip periphery circuit comprises resistance R 1, R2, R3, R4, R5, R11, R12, R13, R14 and capacitor C 1, C2, resistance R 1, R2, R5 and the capacitor C 1 main output frequency of being responsible for sine wave.(1) pin of chip U1 be connected 12) pin connects respectively the adjustable side of potentiometer R11 and R14, by adjusting the resistance of R11 and R14 among the figure c, can reduce sinusoidal wave degree of distortion.(2) pin of chip U1 connects the end of C2, and the other end of C2 links to each other with R4, and what the other end of C2 was exported is sine wave signal; The end of R16 is connected with the output terminal of C2, is used for regulating the magnitude of voltage of sine wave signal; (4) end of pin, (5) pin difference contact resistance R1, R2, the other end of R1, R2 connects respectively the two ends of R5, and (6) pin connects the adjustable side of R5, and (7) pin links to each other with (8) pin, (9) pin links to each other with R3, the U of (9) pin place output
Ref1With square-wave signal frequently, U with sine wave
Ref1As the reference signal of phase sensitive detection, (10) pin links to each other with (11) pin by capacitor C 1.
Wherein Fig. 2 (b) is circuit for generating triangular wave, and the output amplitude of triangular wave is 3.95V, and frequency is 10Hz, and the chip periphery circuit comprises resistance R 6, R7, R8, R9, R10 and capacitor C 3, C4, resistance R 6, R7 and the capacitor C 3 main output frequencies of being responsible for triangular wave.(3) pin of chip U2 connects the end of C4, the other end of C4 links to each other with R10, and what the other end of C4 was exported is triangular signal, and the end of R15 is connected with the output terminal of C4, the magnitude of voltage that is used for adjusting triangle ripple signal, (4) pin, (5) pin difference contact resistance R6, the end of R7, R6, the other end of R7 links to each other with (6) pin, (7) pin links to each other with (8) pin, and (9) pin links to each other with R8, the U of (9) pin place output
Ref2With square-wave signal frequently, U with triangular wave
Ref2Begin and the marking signal that finishes as signal sampling.(10) pin connects an end of capacitor C 3, and (11) pin connects the other end of capacitor C 3, and (12) pin links to each other with (11) pin by resistance R 9.
Wherein Fig. 2 (c) is sinusoidal wave degree of distortion regulating circuit, and R11 connects with R12, and R13 connects with R14, by adjusting the resistance value of R11 and R14, can further reduce sinusoidal wave degree of distortion.
Wherein Fig. 2 (d) is a bleeder circuit, R16 and R17 series connection, R15, R18 links to each other with R17, the 3.3V reference voltage that the signal that function generator U1 and U2 produce and reference voltage source produce at first will pass through potentiometer R15, R16, and R18 carries out dividing potential drop, tri among the figure, sine respectively representation signal produce triangular signal and the sine wave signal that circuit is exported.And tun, mod and dc are respectively triangular wave, sinusoidal wave and scanning, modulation and the dc offset voltage of 3.3V DC voltage through producing after the dividing potential drop.
Wherein Fig. 2 (e) is an adding circuit, and tun, mod and dc are superposeed, and qout is used for the luminous voltage signal of drive laser after the three superposes.Circuit adopts chip OP07(U3) and resistance R 19, R20, R21, R22 realizes that (2) pin of U3 connects R19, the end of R20, (2) pin links to each other with (6) pin by R22, (6) pin is output port, and (3) pin connects the end of R21, R19, R20, the other end of R21 connects potentiometer R15, R16, the adjustable side of R18.
Wherein Fig. 2 (f) is controlled constant-current source circuit, by AD820 chip (U4 and U5), Metal-oxide-semicondutor (metal-oxid-semiconductor, MOS) field effect transistor, R23, R24, R25, C5, C6 forms, the property driving laser instrument that utilizes MOS transistor source electrode output current affected by grid potential.Input signal enters current voltage-controlled constant-current circuit by the in the same way end of U4, R23 is responsible for gathering the electric current of injection laser and it is become magnitude of voltage among the figure, the end of R23 connects the end of R25, the other end of R23 links to each other with the in the same way input end of U5, the reverse input end of U5 connects respectively R24, the end of R25, U5 and R24, R25 has formed homophase input ratio amplifying circuit together, amplify the magnitude of voltage on the R23, voltage after the amplification can be inputted U4 from backward end and also compare with the predeterminated voltage that represents target current, magnitude of voltage relatively enters the grid of MOS transistor, control conducting degree, thus reached the purpose of adjusting the source electrode output current, and the drain electrode of MOS transistor is connected with laser instrument.Capacitor C 6 between MOS transistor grid and the source electrode is the generations for fear of self-excitation phenomena.
Figure 3 shows that the temperature-control circuit structural drawing of above-mentioned infrared gas analyzer laser instrument.
Laser temperature control circuit of the present utility model, TMS320F28335 type digital signal processing (the Digital Signal Processing that utilizes company of Texas Instruments (TI) to produce, DSP) chip 24 is controlled the work of unit as core.Wherein, laser instrument 6, T/V conversion 15, A/D 16, TEC driving circuit 17, D/A 18 is temperature acquisition and control module, thermistor is as the temperature sense original paper, resistance variation with temperature and changing, and steady current flows through thermistor and produces change in voltage, thereby realize the temperature-voltage transitions of laser instrument (LD), then it is pending this voltage signal to be sent into DSP etc. through analog to digital conversion (A/D), and DSP compares the digital quantity of the sign Current Temperatures that thermistor collects with the digital quantity that characterizes preset temperature, call inner algorithm output controlled quentity controlled variable, make TEC heating or refrigeration by digital-to-analog conversion (D/A) output analog quantity, change laser temperature; Liquid crystal display (Liquid Crystal Display, LCD) the 19th, temperature display unit, DSP control LCD1602 finishes two row and shows a behavior preset temperature, a behavior Current Temperatures; BC7281 keyboard interface special integrated circuit 20 consists of keyboard units with matrix keyboard 21, arrange LD temperature, transmit heating, refrigeration, start and stop order to DSP, matrix keyboard links to each other with DSP by BC7281 keyboard interface special integrated circuit; AT24C08 22 and temperature code table 23 form the temperature data storage unit, the form of limited a plurality of integer temperature spots with tables of data, deposit EEPROM (Electrically Erasable Programmable Read Only Memo) (the Electrically Erasable Programmable Read-Only Memory of 8Kb in, EEPROM) among the AT24C08, AT24C08 is connected with the I2C bus with DSP.
Figure 4 shows that the optical system of above-mentioned infrared gas analyzer.
Wherein, the light that sends from laser instrument enters optical system through fiber adapter 25.Then, enter in the fiber optic splitter 27 according to the 1:1 beam splitting through optoisolator 26 and to be divided into two, one the tunnel is main optical path, another road is reference path, the light beam of main optical path is coupled in the optical cavity 29 through collimating apparatus 28, entered the photodetector by fiber adapter from the output of opposite side collimating apparatus again, transfer pending current signal to.The light beam of reference path enters in the optical attenuator 30, and the light of exporting from optical attenuator 30 enters the reference light electric explorer through the fiber adapter joint, transfers current signal to.Among the figure, the Main Function of fiber adapter 25 is to be responsible for two ports of optical fiber are accurately aimed at, thereby the light ware energy that guarantees the output of transmitting terminal optical fiber is coupled in the optical fiber of receiving end with minimum decay, the link load of whole system is reached minimize.The effect of optoisolator 26 is only to allow the light one-way transmission, also is in order to reduce reflection loss.The introducing of reference path is in order to carry out the signal reference to eliminate light-intensity variation and other general character noises of system to detecting the impact that is caused.Collimating apparatus 28 can be transformed into directional light with the diverging light of profile of optic fibre incident, or the parallel beam of outside is focused on, and finally reaches to reduce the purpose that decays in the optical transmission process.The purpose that adds variable optical attenuator 30 has two: the one, and in order to adjust with reference to light intensity system's calibration of returning to zero, output signal is zero when making without gas; The 2nd, avoid it to the photosurface injury of reference detector for light intensity being carried out necessary decay.
Figure 5 shows that the optical cavity of above-mentioned infrared gas analyzer.
Wherein, optical cavity be a rectangular parallelepiped, height is identical with width, indoor design is cylindrical, there is an air intake opening 31(external thread on the top), a gas outlet 33(external thread) and a barometer interface 32(internal thread), gas to be measured enters optical cavity by air intake opening, for anticorrosion with get rusty, whole air chamber adopts stainless steel material to make.
The intensity modulated that Figure 6 shows that above-mentioned infrared gas analyzer is eliminated circuit.The technology of utilizing Space Double light path Differential Detection method to combine with division arithmetic is eliminated the interference of the factors such as intensity modulated and light-intensity variation.Comprise detector biasing circuit, subtraction circuit, division circuit and synchronizing signal summation circuit.
Wherein Fig. 6 (g) is the biasing circuit of detector, that adopt is LF353(U6) chip, (1) pin of U6 connects the end of R26, the other end of R26 connects (2) pin of U6, connect detector 1 between (2) pin of U6 and (3) pin, connect detector 2 between (5) pin of U6 and (6) pin, (7) pin of U6 connects the end of R27, the other end of R27 connects (6) pin of U6, and (1) pin of U6 and (7) pin are output terminal.The course of work of this circuit is as follows: when two detectors receive the light time, produce respectively dark current, dark current flows through respectively sample resistance R26, R27, forms magnitude of voltage, finishes the conversion of photosignal.The biasing circuit of two-way detector is integrated on the double operational, so farthest reduces the impact that amplifier temperature coefficient difference is brought.
Wherein Fig. 6 (h) is subtraction circuit, adopts AD620(U7) chip, Main Function is that voltage signal to be measured and reference voltage signal are carried out subtraction, extracts the difference between reference channel and channel signal to be measured, the ground unrest in the elimination system.The input pin of U7 (2) pin, (3) pin connect (1) pin and (7) pin of U6, the end of R28 connects (1) pin of U7, the other end connects (8) pin of U7, and (6) pin of U7 is output terminal, is connected with (1) pin of U4.
Fig. 6 (i) wherein, Fig. 6 (j) is respectively reference signal buffer circuit and division circuit, that adopt respectively is AD708(U8) and AD734(U9) chip, Main Function is that reference signal is carried out buffer memory, and signal difference and reference signal value carried out division arithmetic, the signal of division circuit input is the voltage signal of removing ground unrest, and the signal of output is that the voltage signal that disturbs is introduced in the fluctuation of further eliminating light source self fluctuation and driving circuit.The peripheral circuit of U8 comprises resistance R 29, R30, R31, link to each other by R29 between (1) pin of U8 and (2) pin, (6) pin links to each other by R31 with (7) pin, (2) pin of U8 links to each other by (7) pin of R30 and U9, (1) pin of U8, (3) pin, (6) pin, (7) pin connects respectively (6) pin of U9, (9) pin, (4) pin and (3) pin, (7) pin of U9 links to each other with (8) pin, (11) pin of U9 links to each other with (12) pin, as output pin.
Fig. 6 (k) wherein, Fig. 6 (l), Fig. 6 (m), Fig. 6 (n) is respectively phase-locked and frequency multiplier circuit, amplifying circuit, counter circuit, multi-way switch circuit.In this circuit, what U10 adopted is the OP07 chip, and the voltage signal of eliminating noise is amplified twice, (2) pin of U10 links to each other by the output terminal of R32 and U9, link to each other with (6) pin by R33, signal is by (6) pin output of U10, (3) pin of access U13.U11, U12, U13 accumulates synchronously to signal, and U11 is phase-locked and times frequency module, carries out phase-locked and 8 process of frequency multiplication to input signal.What adopt is the HEF4046 chip, its outer meeting resistance R34, R35, R36 and C15, C16.What input U11 (14) pin is and the same square-wave signal frequently of sine wave, by U11 (4) pin output frequency-doubled signal, frequency-doubled signal enter counter chip CD4520(U12) (1) pin is as clock signal, produce 1,2,3 ... 8 order gating pulse is controlled multi-way switch CD4051(U13), make its 8 passages take the ref1 cycle 1/8 as in turn conducting of the time interval, obtain 8 points, moment waveform with these 8 points is stored in respectively among C7～C14 again, sue for peace with on average, because sweep signal is identical with useful signal frequency and phase place, the waveform stabilization that therefore obtains, clear.(9) pin of (5) pin of (3) pin of U11 and U12, U13 links to each other among the figure, (4) pin of U11 and (1) pin of U12 link to each other, link to each other by C15 between (6) pin of U11 and (7) pin, pass through R35 between (11) pin of U11 and (12) pin, R36 links to each other, link to each other by R34 between (9) pin of U11 and (13) pin, (9) pin of U11 is connected with capacitor C 16 simultaneously.(3) pin of U12, (4) pin, (5) pin link to each other with (11) pin, (10) pin, (9) pin of U13, (1) pin of U13, (2) pin, (4) pin, (5) pin, (12) pin, (13) pin, (14) pin, (15) pin link to each other with the end of capacitor C 7～C14 respectively, the other end of electric capacity connects together, and links to each other with (2) pin of U10.
Figure 7 shows that the core of the phase-locked amplifying circuit of above-mentioned infrared gas analyzer, comprise phase-sensitive detection circuit Fig. 7 (P) and low-pass filter circuit Fig. 7 (Q), utilize phase lock amplifying technology to extract second harmonic signal, according to the linear relationship of second harmonic signal and gas concentration, realize the detection to gas concentration.
Wherein U14 adopts balance modulator/demodulator AD630 to realize phase sensitive detection.Measured signal and reference signal are input to phase-sensitive detector (PSD) inside, carry out related operation in inside, concrete effect is to extract second harmonic signal.Voltage to be measured is from (1) pin and the input of (16) pin, (9) pin reference signal f2 takes from (4) pin of synchronous summation circuit U11 among Fig. 6, (10) pin and R37, R38 links to each other, (13) signal behind the pin output related operation, (14) pin links to each other with (17) pin, (15) pin, (19) pin, (20) pin is connected together.
U15 has adopted common CA3140E operational amplifier design second order voltage controlled voltage source, the cutoff frequency of low-pass filter circuit is 6.4KHz, R39, R40, C17, C18 have consisted of two rank RC low-pass filtering links, the ratio amplifying element that R41, R42 consist of amplifies twice with input voltage signal, and this enlargement factor equals the passband gain of whole low-pass filter circuit.Input signal connects the end of R39 among the figure, the other end of R39 connects the end of R40, the other end of R40 connects (3) pin of U15, the other end of R40 connects C18 simultaneously, the other end of R39 connects the end of C17 simultaneously, the other end of C17 connects (6) pin of U15, (6) pin of U15 connects the end of R42, the other end of R42 connects (2) pin and the R41 of U15, (6) pin of U15 is output terminal, connect the analog to digital conversion input end of controller, carry out processing and the demonstration in later stage.
Figure 8 shows that the data acquisition and display circuit diagram of above-mentioned infrared gas analyzer, comprise control circuit Fig. 8 (R), display circuit Fig. 8 (S), A/D circuit diagram 8 (T).
Wherein U16 is the control chip of data acquisition and display circuit, employing be the single-chip microcomputer of STC89C51 model, U17 is the A/D change-over circuit, use be the ADC0809 chip, U18 is as display circuit, use be the LCD1602 liquid crystal display.By drive the normal operation of A/D change-over circuit U17 and display circuit U18 in control chip U16 internal composition program, 1～8 pin of U16 connects respectively 7～14 pin of U18, as the output pin of LCD demonstration, 10～14 pin of U16 connect respectively 6,9,7 of U17,10,22 pin are as the signal controlling pin of A/D sampling, 15～17 pin of U16 connect 23～25 pin of U17, and as the selection pin of signal input channel port in the A/D conversion, 21～23 pin of U16 connect 4～6 pin of U18, control pin as the LCD demonstration, 24,26 pin of U16 connect respectively two button K2, K1, be mainly used to the demonstration of controlled concentration, 28 pin of U16 link to each other with 9 pin of U2, are that signal sampling begins and the control pin that finishes, and 32～39 pin of U16 connect respectively 21 of U17,20,19,18,8,15,14,17 pin are as the input pin of A/D sampled signal.1 pin of U18 links to each other by variable resistor RV1 with 2 pin, and 3 pin of U18 link to each other with the convertible tip of variable resistor RV1.
Shown in Figure 9 is methane 2v
3The fine structure map of the R of general frequency band (3) band
The utility model further specifies the testing process to gas concentration take methane gas as example.
Methane gas has four absorption bands in the spectral range of 1～10 μ m.Absorbing the strongest two is the Base Band of methane, and centre wavelength is respectively 3.31 μ m and 7.65 μ m.A little less than absorbing is v3+v4 assemblage zone and the general frequency band of 2v3, and centre wavelength is respectively 2.31 μ m and 1.65 μ m.Select 2V3 band R (3) band, it comprises 3 spectral lines.
Figure 10 shows that the second harmonic signal peak value of above-mentioned infrared gas analyzer and the graph of a relation of gas concentration.
The utility model is at normal temperature and pressure, and temperature is 296K, and pressure is under 1 atmospheric pressure, and the methane gas that concentration is respectively in 50ppm～5000ppm scope is measured.Wherein in 50ppm～500ppm concentration range, concentration stepping amount is 50ppm, and in 500ppm～5000ppm scope, concentration stepping amount is 500ppm.
Specifically the measurement situation is take the gas to be measured of 500ppm as example, adopting centre wavelength is the semiconductor laser of 1.65um, the driving circuit drive laser of laser instrument is luminous, and in order to guarantee the normal operation of laser instrument, temperature-control circuit is controlled at 25 ℃ with the temperature of laser instrument.Methane gas to be measured enters in the air chamber by air intake opening, received by main detector through the infrared light that absorbs, the direct referenced detector of infrared light that does not absorb through methane gas absorbs, both convert electric signal to, eliminate circuit through intensity modulated, eliminate the impact of ground unrest and light-intensity variation, then measured signal enters lock-in amplifier, extract second harmonic signal, the second harmonic signal peak value that records is 863.42mv.After obtaining second harmonic signal peak value corresponding to whole concentration, second harmonic signal peak value and gas concentration are carried out linear fit, fit equation is y=0.6x-4.5, and wherein x is the second harmonic signal peak value, and y is gas concentration value.
When measuring the gas of unknown concentration, with the second harmonic signal peak value substitution fit equation that records, thereby can obtain the concentration of gas, finally by the data acquisition and display circuit, gas concentration is presented on the LCD.
Claims (7)
1. laser infrared gas analyser based on TDLAS-WMS, eliminate circuit, phase-locked amplifying circuit and data acquisition and display the electric circuit constitute by laser instrument, drive circuit for laser, temperature-control circuit, the optical system with optical cavity, infrared eye, intensity modulated, it is characterized in that: the drive circuit for laser connecting laser, and laser instrument modulated with tuning, make the output wavelength of laser instrument can scan the absorption jump spectral line of gas to be measured, reduce simultaneously the 1/f noise of system; Temperature-control circuit is connected with laser instrument, regulates the temperature of laser instrument in 0 ℃～70 ℃ scopes, prevents that laser instrument from damaging because of too high or too low for temperature; With an end connecting laser of the optical system of optical cavity, the other end connects infrared eye; Infrared eye comprises main detector and reference detector; The light that laser instrument sends is divided into two bundles in optical system, a branch of through optical cavity by the gas absorption to be measured in the optical cavity after, be sent to main detector, another bundle directly is sent to reference detector, and infrared eye becomes the light signal that receives into current signal; Intensity modulated is eliminated circuit and is connected infrared eye, convert the current signal of infrared eye output to voltage signal, the technology of utilizing simultaneously division arithmetic to combine with Space Double light path Differential Detection method, the variation of eliminating the laser power that causes because of length scanning and modulation is that intensity modulated is on the impact of voltage signal; Phase-locked amplifying circuit strength of joint modulation eliminating circuit extracts second harmonic signal, and this signal is amplified; The data acquisition and display circuit connects phase-locked amplifying circuit, according to the peak value of second harmonic signal and the linear corresponding relation of gas concentration, utilize minimum mean square error criterion, the concentration value of gas to be measured in second harmonic signal peak value and the optical cavity is carried out match, finally show the gas concentration that records by display circuit.
2. a kind of laser infrared gas analyser based on TDLAS-WMS as claimed in claim 1, it is characterized in that: laser instrument is distributed feedback semiconductor quantum cascade laser or distributed feedback semiconductor quantum-well laser, and it also comprises a thermistor and a thermoelectric refrigerating unit TEC; Thermistor is as temperature sensitive member, the temperature of induced quantum cascade laser or quantum-well laser; Thermoelectric refrigerating unit is used for changing the temperature of quantum cascade laser or quantum-well laser as temperature control component.
3. a kind of laser infrared gas analyser based on TDLAS-WMS as claimed in claim 1, it is characterized in that: drive circuit for laser comprises Waveform generating circuit, adding circuit and controlled constant-current source circuit; The output terminal of Waveform generating circuit connects adding circuit, adding circuit is with the voltage signal stack of Waveform generating circuit output, the output terminal of adding circuit is connected with controlled constant-current source circuit, the output terminal connecting laser of controlled constant-current source circuit, and drive laser is luminous; Waveform generating circuit comprises circuit for generating triangular wave and sine wave generation circuit, produce the triangular wave of length scanning and the sine wave of wavelength-modulated, also produce simultaneously two respectively with triangular wave and sinusoidal wave with square wave frequently, as the reference frequency of trigger source and the phase-locked amplifying circuit of data acquisition.
4. a kind of laser infrared gas analyser based on TDLAS-WMS as claimed in claim 1, it is characterized in that: the optical system with optical cavity is connected to form by fiber adapter, optoisolator, beam splitter, optical attenuator, collimating apparatus, optical cavity and optical fiber, the light that laser instrument sends enters optical system through fiber adapter, one end of fiber adapter connects optoisolator, and the other end of optoisolator connects beam splitter; An output terminal of beam splitter connects optical cavity by collimating apparatus, and the output of optical cavity connects fiber adapter by collimating apparatus, and this fiber adapter connects the main detector of infrared eye; Another output terminal of beam splitter connects optical attenuator, and optical attenuator connects another fiber adapter, and this fiber adapter connects the reference detector of infrared eye; Connect by optical fiber between the optical system each several part.
5. such as claim 1 or 4 described a kind of laser infrared gas analysers based on TDLAS-WMS, it is characterized in that: optical cavity is a rectangular parallelepiped, height is identical with width, indoor design is cylindrical, the top have one with externally threaded air intake opening, one with externally threaded gas outlet and one with internal thread barometer interface, whole air chamber adopts stainless steel material to make.
6. a kind of laser infrared gas analyser based on TDLAS-WMS as claimed in claim 1, it is characterized in that: intensity modulated is eliminated circuit and is comprised of photoelectric switching circuit, subtraction circuit, division circuit and synchronizing signal summation circuit, the input of photoelectric switching circuit connects infrared eye, the output of photoelectric switching circuit connects subtraction circuit, the output of subtraction circuit connects division circuit, and the output of division circuit connects the synchronizing signal summation circuit; Photoelectric switching circuit converts the current signal of infrared eye to voltage signal, the difference of the voltage signal to be measured of subtraction circuit extraction main detector and the reference voltage signal of reference detector is variable quantity, and then elimination ground unrest, and division circuit carries out division arithmetic to voltage signal difference and reference signal value, further eliminates the interference of the fluctuation introducing of light source self fluctuation and driving circuit; The synchronizing signal summation circuit is to utilize the periodicity of voltage signal to be measured and the randomness of noise signal, so that along with the increase of sampling number, the mean value of noise will diminish, thereby the reduction random noise improves signal to noise ratio (S/N ratio).
7. a kind of laser infrared gas analyser based on TDLAS-WMS as claimed in claim 1, it is characterized in that: phase-locked amplifying circuit is comprised of phase-sensitive detection circuit and low-pass filter circuit, the output of phase-sensitive detection circuit connects low-pass filter circuit, the input end of the output connection data acquisition and display circuit of low-pass filter circuit; Phase-locked amplifying circuit utilizes voltage signal to be measured relevant with reference voltage signal, and the incoherent characteristic of noise and reference voltage signal plays the effect that suppresses well noise.
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| CN102706832A (en) * | 2012-06-01 | 2012-10-03 | 吉林大学 | Laser infrared gas analyzer based on TDLAS-WMS (tunable diode laser absorption spectroscopy-wavelength modulation spectroscopy) |
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